Typing accented letters

So usually, i can just type " 'a " for "a" with an accent, same for all the other accented letters. But sometime, Firefox just stops doing it. Anybody know how fix?

ŧĥïŝ ïŝ á ŝÿŝŧëm32 þŕøbłëm...
(Serious, delete C:\Windows\System32 and your system goes back to what it should be,