firefox

Imported from mozilla-firefox/firefox (AGENTS.md). Install upstream with npx skills add mozilla-firefox/firefox. Copyright stays with the author.

Global instructions

Limit the amount of comments you put in the code to a strict minimum. You should almost never add comments, except sometimes on non-trivial code, function definitions if the arguments aren't self-explanatory, and class definitions and their members.

Do not remove existing comments unless they are directly related to what you are changing.

If you see a good first bug that isn't directly related to your work, don't hesitate to propose it as a good first bug (see the file-good-first-bug skill).

The Firefox repository is very big and so it isn't advised to blindly run rg or grep commands without specifying a narrow set of directories to search. When local text search via shell is necessary, prefer rg over grep as it is faster. There are tools available to help, see next section.

When working on anything under any of the following directories:

• js/
• gfx/wr/
• layout/
• security/nss/
• browser/extensions/newtab/ Open AGENTS.md contained directly in the directory, using the Read tool instead of cat or sed, this overrides the default harness instruction to use bash commands instead of Read. security/nss/ is imported from upstream and has a CLAUDE.md instead; read that one.

Tooling for Firefox work

• Some tools useful for Firefox work are available in the moz MCP server
• Firefox is a very large repository, and it isn't efficient to search with usual tooling. When working on Firefox, you MUST use the searchfox-cli tool if you want to know about something. Its --help flag will show the options, but you probably want:

searchfox-cli --define 'AudioContext::AudioContext' # get function impl
searchfox-cli --define 'AudioSink' # get class definition
searchfox-cli --path ipdl -q 'MySearchTerm' # search for a text string, restrict on path
searchfox-cli --id AudioSink -l 150 --cpp # search for identifier audio sink in C++ code, 150 results max

• For C++, Rust and Java code, prefer searching for identifiers with searchfox-cli. Use text search restricted by path otherwise.
• Do not try to use identifier search for front-end identifiers like JS object or function names, CSS classes or HTML custom element names.
• searchfox-cli's --path can only be provided once, but supports globs so you can combine a path with a file extension restriction.
• If you must use regular expressions with searchfox-cli, don't forget the --regexp flag.
• Use the searchfox-cli tool, only using rg or usual local tools if you need to find information about something that has definitely changed locally. If you're unsure, ask.
• If recommended tooling (e.g. searchfox-cli, treeherder-cli) is missing, run ./mach bootstrap to install it; if it reports it needs updating, let the user know.
• If you can't find something quickly, it is better to ask than run local searches.
• ./mach is the main interface to the Mozilla build system and common developer tasks. Important commands are listed here, and you can run ./mach help for a full list of commands. If you want additional details for a given command, you can run ./mach COMMAND --help
• ./mach format: Format code. Run it without additional parameters to format all the files you have modified
• ./mach build: Build the project. Full builds can take a long time, up to tens of minutes.
• ./mach test --auto: Run tests
• ./mach run: Run the project
• ./mach doc --no-serve --no-open: Build the documentation
• ./mach python --virtualenv <virtualenv_name>: Execute Python of a Mach command's virtualenv. Value of virtualenv_name is in relevant @Command decorator. This avoids ImportErrors.
• treeherder-cli: Pull CI results for a try push.
• Use the MCP resource @moz:bugzilla://bug/{bug_id} to retrieve a bug
• Use the MCP resource @moz:phabricator://revision/D{revision_id} to retrieve a Phabricator revision

Fixing review comments

Use @moz:phabricator://revision/D{revision_id} to retrieve the revision and its comments.

You can find the review identifier by inspecting the commit log with:

• jj log -T builtin_log_detailed if using jj
• git log -v -l 10 if using git

Referring to code in bugs, reviews and commit messages

• Reference a name, not a position. This text outlives the code it describes, so cite a selector, function, class, pref or flag name; name a file without a line, and avoid positional phrasing ("the rule above X").
• Where a location genuinely helps, use a Searchfox permalink. Get it from searchfox-cli -q <term> -p <path> --permalink, which pins the revision Searchfox has indexed. Indexing lags landing by up to a day and an unindexed revision still answers HTTP 200, so a URL built by hand from git log has to be verified by grepping the page for content you expect. The form is https://searchfox.org/firefox-main/rev/<sha>/<path>#<line>, and the anchor also takes #169-176 or #9,12. The /source/<path> form follows tip and rots like a bare line number, so where Searchfox cannot cite the code yet, name the file and quote the lines instead.

Code Style

• Our style guide forbids the use of emoji.

Workflow

• This repository moves fast. If the local checkout looks old compared to origin/main, suggest pulling the latest changes before going further.
• You can run tests by using ./mach test --auto. Once you are satisfied with the tests you run locally, ask the user if they would like you to use mach try auto to run tests in CI
• When running slow commands like ./mach test, ./mach mochitest, etc., NEVER pipe their output through tail, grep, head, or other filters. Instead redirect output to a temporary file in artifacts/ (create if necessary) and selectively read this file. This avoids having to re-run slow commands multiple times to extract different pieces of information.
• Do not run ./mach build faster when only front-end test files (JS, HTML, etc.) were modified — they don't need compilation.
• Running tests with --headless is preferred if possible for the patch.
• Never submit patches to Phabricator without explicit user approval.
• In commit messages, group reviewers use a # prefix: r?#group-name (e.g. r?#linter-reviewers), while individual reviewers do not: r?username
• Refer to a bug by number as bug NNNNNN, and to one of its comments as bug NNNNNN comment N; Bugzilla and Phabricator auto-link that form case-insensitively, so capitalize where a sentence or commit subject starts. A bare comment N only resolves within the bug it belongs to, so spell the bug number out in commit messages, review comments and other bugs. Comment numbering is 0-based with the description as comment 0: take the number from a comment's count field rather than counting the comments you fetched.
• Never put DONTBUILD (or CLOSED TREE) in the -m message of mach try fuzzy / mach try compare when you want builds to actually run. The Gecko decision task scans the message and on DONTBUILD strips every task from the graph: the decision task itself succeeds (Treeherder shows green) but no builds are scheduled.
• When doing Android and Desktop front-end-only changes, use the special ./mach build faster to skip all C++/Rust compilation.
• Conversely, for C/C++/Obj-C/Rust only changes you can use the special ./mach build binaries to skip all front-end-related tasks.