Suddenly I can't type in Firefox. Keyboard doesn't have any issue as everything works fine on other applications. I have tried disabling add-ons, restarting FF in safe mode, resetting Firefox, all with no success. My Firefox is version 26.0.
Try using a previous version of FF, it can be that there was a Windows update that somehow messed something up with that version of FF. My PC usually starts acting weird after a Windows update. I then do a system restore from before the update and then all is well again.