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 legacy Python version of the validator for the addons.mozilla.org website. The repository is on GitHub. The preferred bug tracker is GitHub although components do exist on bugzilla for legacy bugs. Documentation is on readthedocs.