Did you check the "Reopen windows when logging back in" setting?

See also https://discussions.apple.com/thread/3193281

Nope, Firefox was not even open

Did you try to remove the mentioned plist file(s)?

And just which plist file is that. The thread has over 9 pages of replies with all kinds of "remove this plist"

Any other thoughts

Would this have anything to do with why it would try and autostart?

6/20/13 1:43:08.972 PM WindowServer[89]: reenable_update_for_connection: UI updates were finally reenabled by application "Firefox" after 3.48 seconds (server forcibly re-enabled them after 1.00 seconds)

Let's not have two threads on the same subject please.

https://support.mozilla.org/en-US/questions/961315