Re: Suddenly, my keyboard doesn't work with Mozilla Firefox. I reset Firefox, but still the keyboard doesn't work. I'm typing this now from Internet Explorer.
It is possible that you have switched the keyboard layout by accident to one that isn't working.
Windows remembers the keyboard layout setting per application and you may have changed the keyboard layout by accident via a keyboard shortcut.
Make sure that you have the Language bar visible on the Windows Taskbar.
You can do that via the right-click context menu of the Taskbar: Toolbars > Language Bar.