I'm having trouble reproducing this manually, but I could see pretty clearly when writing xpcshell tests for bug 557710 that temporary add-ons were not revealing the correct add-on in a test like this: 1) run Firefox with build in system add-on 2) install a regular add-on with same ID 3) load temporary add-on again with same ID 4) remove temporary add-on Expected: The regular add-on is revealed Actual: The built-in system add-on is revealed It also looked like even the system add-on was not reactivated if AddonManager was shut down immediately after step #4. In both cases, I could see in the log output that the underlying add-ons were still present in the addons database. If I `touch`'d one of the XPI files and re-started, then they would be activated again. I think this might be happening because the TemporaryInstallLocation's uninstallAddon method does nothing, while the MutableDirectoryLocation (used for regular add-ons) does this: https://dxr.mozilla.org/mozilla-central/rev/a551f534773cf2d6933f78ce7d82a7a33a99643e/toolkit/mozapps/extensions/internal/XPIProvider.jsm#5143

Per policy at https://wiki.mozilla.org/Bug_Triage/Projects/Bug_Handling/Bug_Husbandry#Inactive_Bugs. If this bug is not an enhancement request or a bug not present in a supported release of Firefox, then it may be reopened.