For some time (maybe since Firefox 4.0), I think there is a bug in remembering my preferred locale. And my recent forays in PHP seems to confirm it: it is NOT defined.
The problem exists when I use the French portable edition of Firefox 4.01 and now 5.0, on Windows 7 and, I'm almost certain, on Windows XP. And it definitely is unique to the Portable edition of Firefox because the non-portable edition does not have this problem. And starting a new profile doesn't solve the problem.