The problem is with Firefox's password memory system. Turn it off - what it's doing is, when you load the update.bml page (which means yes, if you use a client to post you'll also not have the problem), there is a section for "update as other user". Firefox sees the "username" and "password" boxes and automatically fills them in. That makes IJ think that you're posting as another user, and as you're "not logged in" as the "other user", it doesn't have your community membership information.
If you go to the Preferences and find the password information and take out InsaneJournal it'll be fine.