Since the last update [76.0 (64-bit)], whenever I enter a '''fullscreen''' mode in any media player on a website (e.g. Youtube) I can no longer '''Alt-tab''' normally, even if I close the fullscreen. Instead it takes 3 times to switch to another window - first press of the Alt-tab combination causes Firefox to lose focus but remain on top, 2nd press returns focus onto Firefox and then the 3rd one actually switches to another window. This also occurs when simply switching to fullscreen on any other website as well (by pressing F11). This also causes any newly open windows to be displayed under the Firefox window. Pressing Ctrl-Alt-Esc minimizes the window normally and the issue does not occur then, however, it occurs again whenever I go fullscreen. '''The problem occurs also in the Safe mode.''' Antivirus did not find any issues, so I don't believe the cause to be external. OS is Windows 10 Home, 64-bit.