This happens when you close, then quickly re-open Firefox normally.
Firefox takes a few seconds to fully close.
The other cause is you close it but the close hangs
Two solutions.. If its just a hot-switch issue (Close and re-open swiftly) count to Ten, try again, (Dang, made it rhyme)
OR CTRL-SHIFT-ESCAPE, find Firefox on the re****ing pop up, right clck and choose End task.
Last time it happened to me was... Today (Swift close/re-open, waited and tried again, worked) Toshiba 8.1, Dell xp, Other XP all the same.