Mozconfig Skill

This skill generates correct, complete Firefox mozconfig files for various build configurations. Mozconfigs control build options like debug mode, sanitizers (ASan/TSan), optimization levels, and more.

When to Use This Skill

Use this skill when the user:

• Asks to create a mozconfig file
• Wants to set up a specific build configuration (debug, ASan, TSan, etc.)
• Wants to replicate a try server or CI build configuration locally
• Needs help configuring Firefox builds
• Mentions build types like "ASan build", "debug build", "fuzzing build", etc.

What You Will Do

Follow these steps in order. This skill is interactive - use AskUserQuestion to gather requirements and preferences from the user.

Step 1: Detect Platform

Detect the user's operating system and architecture.

1. Run these commands to detect the platform:

   uname -s  # Operating system
   uname -m  # Architecture

2. Map the results:
   • Linux + x86_64 → linux64 (all configs supported)
   • Darwin + x86_64 → macosx64 (no TSan)
   • Darwin + arm64 → macosx64-aarch64 (no TSan)
   • MINGW*/MSYS*/CYGWIN* + x86_64 → win64 (ASan 64-bit only, no TSan)
3. Report to user: Tell them what platform was detected and any limitations:
   • Linux: All configurations supported
   • macOS: TSan not supported
   • Windows: ASan 64-bit only, TSan not supported
4. Note any special platform constraints for later steps.

Step 2: Determine Configuration Mode

Ask the user how they want to configure their build.

Use AskUserQuestion to present three options:

Question: "How would you like to configure your Firefox build?"
Header: "Config mode"
Options:
  1. "Common preset" - "Pre-built configurations for common use cases (debug, ASan, TSan, etc.)"
  2. "Try server match" - "Replicate a CI/try server configuration locally"
  3. "Custom configuration" - "Interactive guided configuration for specific needs"

Based on their choice, proceed to Step 3A, Step 3B, or Step 3C.

Step 3A: Preset Mode

For users who selected "Common preset".

Present Platform-Appropriate Presets

Use AskUserQuestion to show available presets. Filter by platform - don't show TSan on macOS/Windows, for example.

Question: "Which preset configuration do you want?"
Header: "Build preset"
Options (filter by platform):
  1. "Debug" - "Standard debug build with assertions and symbols"
  2. "Debug (no optimization)" - "Debug build with --disable-optimize (very slow but best for debugging)"
  3. "ASan optimized" - "Address Sanitizer with optimization (recommended for testing)"
  4. "ASan debug" - "Address Sanitizer with debug mode (better for debugging ASan findings)"
  5. "TSan" - "Thread Sanitizer (Linux only)" [only show on Linux]
  6. "Non-unified build" - "Disable unified compilation (slower build, finds hidden dependencies)"
  7. "Artifact build" - "Frontend-only development (downloads pre-built C++/Rust)"
  8. "Code coverage" - "Instrumented build for code coverage analysis (Linux)" [only show on Linux]
  9. "Fuzzing + ASan" - "Combined fuzzing and ASan support (for fuzzing workflows)"
  10. "Valgrind" - "Optimized for running under Valgrind (Linux)" [only show on Linux]

Generate Mozconfig from Preset

1. Read the reference file: Open .claude/skills/mozconfig/references/build-options-reference.md
2. Find the preset section: Look for the preset they selected in section "1. Preset Mozconfigs"
3. Extract the mozconfig: Copy the complete mozconfig for that preset
4. Apply platform-specific adjustments:
   • For ASan on Linux, uncomment the --enable-valgrind line
   • For ASan debug on Linux, uncomment --enable-valgrind and --enable-gczeal
   • Ensure Windows configs don't have Linux-only options
   • Ensure macOS configs have appropriate SDK if needed
5. Proceed to Step 4 to write the file.

Step 3B: Try Server Match Mode

For users who selected "Try server match".

Ask Which Try Configuration to Match

Use AskUserQuestion to determine the target configuration:

Question: "Which try server configuration do you want to replicate?"
Header: "Target platform"
Options:
  1. "Current machine" - "Match a try config for my detected platform (recommended)"
  2. "linux64" - "Linux 64-bit configuration"
  3. "win64" - "Windows 64-bit configuration"
  4. "macosx64" - "macOS x86_64 configuration"
  5. "macosx64-aarch64" - "macOS ARM64 configuration"

Then ask for build type:

Question: "Which build type from try server?"
Header: "Build type"
Options (examples, adjust based on platform):
  1. "debug" - "Standard debug build"
  2. "debug-asan" - "Debug with Address Sanitizer"
  3. "nightly-asan" - "Optimized Address Sanitizer build"
  4. "tsan" - "Thread Sanitizer (Linux only)"
  5. "code-coverage" - "Code coverage instrumentation"
  6. "debug-fuzzing" - "Debug with fuzzing support"
  7. "nightly" - "Nightly release configuration"
  8. "plain-opt" - "Plain optimized build"

Resolve and Flatten the Mozconfig

Now you need to read and flatten the try server mozconfig files.

1. Start with the base file: Read browser/config/mozconfigs/<platform>/<type>
   • Example: browser/config/mozconfigs/linux64/debug-asan
2. Follow all includes recursively: When you see . $topsrcdir/path/to/file or . "$topsrcdir/path/to/file", read that file too
3. Build a complete mozconfig by concatenating all the files in order
4. Strip CI-only lines: Remove lines that reference:
   • $MOZ_FETCHES_DIR (CI artifact fetches)
   • $topsrcdir/browser/config/mozconfigs/common (CI-only base)
   • $topsrcdir/build/mozconfig.common (contains CI exports)
   • $topsrcdir/build/mozconfig.automation (CI-only)
   • $topsrcdir/build/mozconfig.cache (CI-only)
   • $topsrcdir/build/mozconfig.common.override (CI-only)
   • AUTOCLOBBER=1 (CI auto-clobber)
   • MOZ_AUTOMATION_* exports
   • MOZ_PKG_SPECIAL exports (optional)
   • MOZ_PACKAGE_JSSHELL (optional, but CI-specific)
   • MOZ_INCLUDE_SOURCE_INFO (CI-only)
   • MOZILLA_OFFICIAL=1 (CI-only)
   • API keyfile paths (--with-*-keyfile)
   • Branding paths referencing automation
   • Lines setting LLVM_SYMBOLIZER to $MOZ_FETCHES_DIR paths
   • Sccache cloud configuration
5. Keep important lines:
   • All ac_add_options (except keyfiles and automation paths)
   • RUSTFLAGS exports
   • unset RUSTFMT
   • Debug/optimization settings
   • Sanitizer configuration
6. Replace CI paths with local equivalents:
   • Instead of $MOZ_FETCHES_DIR/llvm-symbolizer/bin/llvm-symbolizer, use a comment suggesting to set ASAN_SYMBOLIZER_PATH or TSAN_SYMBOLIZER_PATH environment variable
7. Add helpful comments: Explain what each section does
8. Proceed to Step 4 to write the file.

Step 3C: Custom Mode

For users who selected "Custom configuration".

Guide User Through Configuration Options

Use AskUserQuestion iteratively to build the configuration. Read the reference file first to understand available options.

Question 1: Optimization Level

Question: "What optimization level do you want?"
Header: "Optimization"
Options:
  1. "Default optimization" - "Standard -O2/-O3 (fast, production-like)"
  2. "Minimal optimization (-O1)" - "Useful for debug+opt builds"
  3. "No optimization" - "Slowest, but easiest to debug"

Question 2: Debug Mode

Question: "Enable debug mode with assertions?"
Header: "Debug mode"
Options:
  1. "Yes" - "Enable debug assertions and runtime checks"
  2. "No" - "Optimized build without debug checks"

Question 3: Debug Symbols

Question: "What level of debug symbols?"
Header: "Debug symbols"
Options:
  1. "Line tables only" - "Minimal symbols (smaller binaries, good for sanitizers)"
  2. "Default symbols" - "Standard debug symbols"
  3. "Full symbols (-g)" - "Maximum debug info (large binaries)"
  4. "No symbols" - "Strip all debug symbols"

Question 4: Sanitizer

Question: "Which sanitizer (if any)?"
Header: "Sanitizer"
Options:
  1. "None" - "No sanitizer"
  2. "ASan" - "Address Sanitizer (memory errors)"
  3. "TSan" - "Thread Sanitizer (data races, Linux only)" [only on Linux]
  4. "MSan" - "Memory Sanitizer (uninitialized memory, rarely used)"
  5. "UBSan" - "Undefined Behavior Sanitizer"

If ASan or TSan is selected, automatically add required options:

• --disable-jemalloc
• --disable-crashreporter
• --disable-elf-hack (Linux)
• --disable-profiling
• --disable-install-strip

If TSan is selected, also add:

• --disable-sandbox
• export RUSTFLAGS="-Zsanitizer=thread"
• unset RUSTFMT
• --disable-warnings-as-errors

Question 5: Additional Features

Allow multiple selections:

Question: "Select any additional features you need:"
Header: "Features"
Multi-select: true
Options:
  1. "Fuzzing support" - "Enable libFuzzer integration"
  2. "Code coverage" - "Enable coverage instrumentation"
  3. "Valgrind support" - "Valgrind compatibility (Linux)"
  4. "Disable unified build" - "Build files separately (slower, finds hidden deps)"
  5. "Artifact build mode" - "Download pre-built binaries (frontend-only)"

Assemble the Mozconfig

Based on all answers:

1. Start with MOZ_OBJDIR: Generate an appropriate object directory name based on the config
   • Example: obj-debug, obj-asan, obj-tsan, obj-debug-asan
2. Add optimization flags based on Question 1
3. Add debug flags based on Question 2
4. Add debug symbol flags based on Question 3
5. Add sanitizer flags and required options based on Question 4
   • Use the reference file section "2. ac_add_options Reference" → "Sanitizers"
6. Add feature flags based on Question 5
7. Add platform-specific options:
   • Linux with ASan: consider adding --enable-valgrind
   • TSan: ensure all TSan requirements are present
8. Add helpful comments explaining each section
9. Proceed to Step 4 to write the file.

Step 4: Generate and Write Mozconfig

Final step: create the mozconfig file with user confirmation.

Prepare the Mozconfig Content

1. Add a header comment to the generated mozconfig:

   # Firefox mozconfig
   # Generated by Claude Code mozconfig skill
   # Configuration: [describe the config - e.g., "Debug + ASan optimized build"]
   # Platform: [detected platform]
   # Generated: [current date]

2. Organize the content into commented sections:
   • Object directory
   • Optimization settings
   • Debug settings
   • Sanitizer configuration
   • Additional features
   • Platform-specific settings
   • Environment variables
3. Add usage notes at the end as comments:

   # Build instructions:
   # 1. Save this file as 'mozconfig' in your mozilla-central directory
   # 2. Run: ./mach clobber && ./mach build
   # 3. Run Firefox: ./mach run
   #
   # [Add sanitizer-specific notes if applicable]

Ask Where to Write

Use AskUserQuestion:

Question: "Where should I save the mozconfig file?"
Header: "Output path"
Options:
  1. "./mozconfig (recommended)" - "Standard location in repository root"
  2. "./mozconfig-[configname]" - "Named config file (e.g., mozconfig-asan)"
  3. "Custom path" - "Specify a different location"

Check for Existing File

If the file already exists:

1. Read the existing mozconfig
2. Show a diff: Display what would change (you can describe the changes textually)
3. Ask for confirmation:

   Question: "A mozconfig already exists. How should I proceed?"
   Header: "File exists"
   Options:
     1. "Overwrite" - "Replace the existing mozconfig"
     2. "Write to different path" - "Save with a different name"
     3. "Cancel" - "Don't write the file"

Write the File

1. Write the mozconfig to the chosen path using the Write tool
2. Confirm to user: Report success with the path

Important Warnings

After writing, always warn the user about:

1. Clobber requirement:

   ⚠️  Important: Run `./mach clobber && ./mach build` after creating or changing your mozconfig.

2. Sanitizer-specific warnings if applicable:

   For ASan:

   ASan Notes:
   - Set ASAN_SYMBOLIZER_PATH environment variable to your llvm-symbolizer location
   - On macOS, set MOZ_DISABLE_CONTENT_SANDBOX=1 to symbolize content process crashes

   For TSan:

   TSan Notes:
   - TSan requires a special Rust toolchain. Run:
     ./mach artifact toolchain --from-build linux64-rust-dev
     rm -rf ~/.mozbuild/rustc-sanitizers
     mv rustc ~/.mozbuild/rustc-sanitizers
     rustup toolchain link gecko-sanitizers ~/.mozbuild/rustc-sanitizers
     rustup override set gecko-sanitizers
   - Set TSAN_SYMBOLIZER_PATH to your llvm-symbolizer location (required)
   - On NVIDIA with proprietary drivers, you may need to disable GPU acceleration

3. Build time estimate if appropriate:
   • Artifact builds: "Very fast (minutes)"
   • Optimized builds: "Moderate (15-30 minutes typical)"
   • Debug builds: "Moderate (15-30 minutes typical)"
   • Non-unified builds: "Very slow (can be hours)"
   • Sanitizer builds: "Moderate to slow (20-40 minutes typical)"

Offer MOZ_OBJDIR Customization

Ask if they want to customize the object directory name:

Question: "The object directory is set to [current value]. Would you like to change it?"
Header: "Object dir"
Options:
  1. "Keep default" - "Use [current value]"
  2. "Use config-based name" - "Use a name based on the config (e.g., obj-asan)"
  3. "Custom name" - "Specify a custom object directory name"

If they choose option 2, suggest names like:

• Debug: obj-debug
• ASan: obj-asan
• TSan: obj-tsan
• Debug+ASan: obj-debug-asan
• Artifact: obj-artifact

Step 5: Summary and Next Steps

Provide a clear summary of what was created.

Tell the user:

1. ✅ What was created: "Created [config type] mozconfig at [path]"
2. Next steps:

   Next steps:
   1. Run: ./mach clobber && ./mach build
   2. After build completes: ./mach run
   3. [If sanitizer: Set environment variables as noted above]

3. Offer additional help:

   Need help?
   - Modify the mozconfig file directly to adjust options
   - Run /mozconfig again to create a different configuration
   - Check .claude/skills/mozconfig/references/build-options-reference.md for all options

Reference Files

This skill uses the following reference files:

• .claude/skills/mozconfig/references/build-options-reference.md - Comprehensive reference of all mozconfig options, presets, and platform notes

Always read the reference file when you need to:

• Look up preset mozconfigs
• Understand what options are available
• Check platform-specific constraints
• Generate custom configurations

Error Handling

If something goes wrong:

• File read errors: Gracefully explain that the try server config couldn't be read and suggest using a preset instead
• Invalid options: Warn the user and offer to use a safe default
• Platform incompatibility: Clearly explain why an option isn't available (e.g., "TSan is only supported on Linux")

Additional Notes

• Always be helpful: If the user is uncertain, guide them toward common, safe configurations
• Explain trade-offs: When presenting options, help them understand the implications (speed vs. debuggability, etc.)
• Validate combinations: Don't allow incompatible options (e.g., jemalloc + sanitizers)
• Use the reference: The reference file is comprehensive - trust it and use it
• Platform matters: Always respect platform limitations

Success Criteria

You've successfully completed this skill when:

1. ✅ User's platform is detected
2. ✅ Configuration mode is selected
3. ✅ Mozconfig is generated based on their choices
4. ✅ File is written to their chosen location
5. ✅ User is given clear next steps and warnings
6. ✅ User understands what was created and how to use it