FF 15 Won't stay in normal window mode.
FF 15 is constantly jumping from a normal screen to full screen and when it does I loose my menu bar, Navigation Bar, my tabs and my Task Bar. I have to sit with my finger on F11 so I can switch it back when it jumps to full screen. I have tried setting my options and that doesn't help. I know that it will probably be a year (if ever) before anyone replies to this with any help but it's worth a shot.
You can try to check if this happens with other programs. eg. Internet Explorer. If there are any Windows OS tweaks applied you can also try to make adjustments in the tweaks program. Otherwise you can consider the Reset Firefox feature via Help (Alt + H) > Troubleshooting Information.
(To revert to the previous profile you were using, close the new profile (i.e. exit Firefox), start Firefox and choose the Default User profile. While the Profile Manager is open, you can also delete the newly reset profile (the one containing random numbers), or the former profile, as the case may be). If the reset is okay, it would be a good idea to delete the old profile as otherwise it would be left intact with the previous data until deleted.