If you have more than one layout installed the it can still be a Windows issue because Windows remembers the layout per program and MS has chosen a simple way like Ctrl or Alt + (Left or Right) Shift to cycle the installed layouts. Firefox has a lot of menu items with such a key sequence that easily trigger such a layout switch. It is best to disable that cycle sequence and use a specific shortcut if you want to switch the keyboard layout.

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.

See:

• http://windows.microsoft.com/en-US/windows-vista/The-Language-bar-overview - The Language bar (overview)

OK, I have to admit I was a bit skeptical at first because I've never seen any other app behave like this before, but this worked! I am running Firefox 8, not that it makes a difference.

One other symptom I had was when going into the keyboard language control panel in Windows 7, it took a very long time for that dialog box to appear (over a minute), to a point where I thought I had a virus preventing me from fixing this. But once the system unclogged itself, about 10 or so windows appeared for the keyboard language!