I'm not sure exactly when or for how long this leaks, but attempting to add a new search engine for the search box on about:home leaks the following:
After performing a new Google search, the first compartment disappears but the second remains. There may be other ways to leak as well, but as adding the first search engine failed for me, I've been as of yet unable to fully test the add-on.
Not CCing any of the listed authors, as none have Bugzilla accounts, but they've been notified of this bug via AMO.
Developer chased via info request.
developer has replied to say they expect a fixed release next week.
The 'fixed' release promised unfortunately didn't fix the leak and can still be reproduced with the details in comment 0. I've approved as it addressed some other issues and the developer is aware of the need to fix it in the next release.
developer chased for update via AMO
Developer has replied saying they should have a new version ready next week.
Tested latest version and no zombie compartments were detected.