The BrandMyMail add-on creates zombie compartments for the webmail pages that it manipulates. The steps to reproduce are complex, given that the add-on requires account creation and setup, but it should suffice to attempt to compose an email on a supported site with a valid account and then close the tab. Not CCing developer, as none appear to have Bugzilla accounts, but I've notified them via AMO.

The current version still leaks. I sent a message to the developer.

Latest version doesn't fix the zombie compartments though the developer does seem aware of it. Add-on is already only preliminary reviewed so no threat of downgrade.

Since this add-on has never been fully public and has low usage, I don't think it's worth tracking this. The developer is aware of the problem and is supposedly working on resolving it.