Currently we write extensions.ini on shutdown, if there are pending changes. That slows down shutdown, and could potentially cause issues if the application doesn't cleanly shutdown. Ideally, extensions.ini would be written at the time there's a change (off main thread, of course). The catch: It's currently written on shutdown because we also update addon state in the database on shutdown, thanks to non-restartless changes. Ad while the application is running, we need to know the state of addons *for that session*, not for the next session. But I think we're currently handling that backwards. Rather than keeping current session state on disk, it should be kept in-memory. Next session state should be written to disk immediately.

Would it make sense to add a "pendingChanges" (or some better name) section to the JSON database from bug 853389, so that we can write the data async as soon as it has changed and then merge the data at next startup? That way we don't need to deal with the in-memory and on-disk versions being different.

Yea, that seems like it should work - since its still the next session state persisted on disk immediately, just via a type of transaction log. Will need to be careful to try to guarantee it gets removed on startup, to avoid costs on next startup. I guess that won't incur any more additional IO updating the DB on startup, since in that case we'd need to do it anyway (I think).

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.