Unclosable Window - Possible Exploit or Just Screen Aliasing Problem?

[drbrando007]

Confirmed that it appears to be firefox-bin that does not close this window. It appears the process will exit correctly if you quit the program from the File menu option, this solution is unconfirmed as of yet.
Currently checking to see that if while the phantom window remains open is the profile still accessible by the process, possibly indicative of a route for data leakage.

[FuturePilot]

Confirmed that it appears to be firefox-bin that does not close this window. It appears the process will exit correctly if you quit the program from the File menu option, this solution is unconfirmed as of yet.
Currently checking to see that if while the phantom window remains open is the profile still accessible by the process, possibly indicative of a route for data leakage.

It could be caused by an addon. Try running Firefox in Safe Mode.
It could also be a problem with your profile. Temporarily move your profile and let Firefox create a fresh one. See if it's reproducible.