I have customized my French keyboard on Windows (7) using "microsoft keyboard layout creator". In particular, I assigned some ctrl+key combinations to generate some characters. For instance, ctrl+= generates the character "#". That worked perfectly until Firefox 49 (I think). But since version 50, typing "ctrl+=" does not do anything. I have tried to see what happens by using the Menu Wizard extension, and it seems that when I press "ctrl+=", Firefox now sees "ctrl+#", so "ctrl" + the generated character. This is true for all my "ctrl+key" = character redefinitions. This is definitely a bug: either you see "ctrl+=" (but you shouldn't, since the keyboard sends a character), or you see the character "#" (which Firefox did very well until recently). The fact that the ctrl key is down when you receive the character "#" does not mean I press "ctrl+#" (in that case, you don't receive a character).