As on topic. The problem appeared simultanously on multiple computers with various systems (Windows XP, Windows 7), after last update to Firefox 46.0 with some more additions update (I don't remember exactly which additions, as it is automatic - but I suppose it was "Video DownloadHelper" or "Xmarks"). On correctly working system - Firefox and only Firefox doesn't reply on keyboard keys - with a cursor in the address box - when pressing keys - the characters doesn't show in the line. Nor the search box. First I found the easiest workaround: reboot the computer and everything works. Then I switched on another remote system, accessed via RDP - with the same problem. But there it was not so easy to reboot (working production services etc.). So I started to search for a solution in mozilla support. I found the following archived threads about this: https://support.mozilla.org/pl/questions/952115 https://support.mozilla.org/pl/questions/1020654 https://support.mozilla.org/pl/questions/1056923 https://support.mozilla.org/pl/questions/1077142 so I guess, the problem is known from at least 2 years and is returning. More than 100 persons confirmed the problem in above threads. I tried one of the suggested solutions: I pressed WinKey + LeftShift - and it worked! (even via RDP). The question is: what is the reason of the problem? How to avoid it for the future? Somebody has any suggestions? Best regards, Lukasz