Browser code is maintained in mozilla-central. This includes several areas related to add-ons. All of the following bug trackers are in Bugzilla:
Handles installing, running and updating add-ons within the application (eg: Firefox). Also has pages like about:addons. In bugzilla - product: Toolkit, component: Add-ons Manager. Existing bugs or file a new bug.
Almost everything else is on GitHub and issues are tracked in GitHub. This is a non-exhaustive list. Other repositories and libraries do appear around these main libraries:
In the past this repository has been known as remora, zamboni or olympia.
The linter checks WebExtensions for common errors and potential problems. It is used on addons.mozilla.org and web-ext. It can also be run in stand-alone mode. The repository, issue tracker and documentation is on GitHub.