UI responsiveness is when you click or hit a key and the app responds immediately, or does not while it should. Goal for "responsive": UI should react within 20ms, in any situation. -- (Bug 384323 is about the core platform.) This bug is about the applications (Thunderbird, Firefox UI etc.), where they need to change to make this possible. For example, if Thunderbird's junk filter or "move message to folder" function is written so that it blocks the main thread, and Thunderbird code needs to be fixed for that, this is part of this bug.

There's an existing meta bug 91351, marking it as blocker (bug 384323 comment 3 / 4)

bug 159909 should block this or bug 384323?

Per title, probably this, bug 261633 is what has to be fixed, it is all interdependent anyway..

anything where webpages hang the browser should block 384323 (core), not this. There's nothing that the Firefox UI can do about it. The only browser bugs which should block this are when e.g. reading bookmarks blocks the rest of the UI.

I'll take this and look through dependencies to see if they are snappy-worthy

Marking all tracking bugs which haven't been updated since 2014 as INCOMPLETE. If this bug is still relevant, please reopen it and move it into a bugzilla component related to the work being tracked. The Core: Tracking component will no longer be used.