Ubuntu 12.04, Firefox 13.0, xfce
For some reason, my Firefox installation has stopped paying attention to ALT keystroke combinations. That is, when I try an ALT-something key combination, I get the "something" as if ALT wasn't pressed at all. The "ALT" mechanism works fine in all the other apps I've tried; it used to work in Firefox (like, literally for years and years, until just the other day).
I've tried running on a new Firefox profile, and it behaves the same way, so I don't think it's a corrupted profile. I've got another laptop, different hardware, but essentially the same setup as this one, and Firefox and the "ALT" key get along fine there.
I have no idea where to even start trying to diagnose this. The general issue of keystroke processing on Linux is, I know, a fantastically complicated area, but since my "ALT" key seems to work perfectly well in every other application on the computer, I'm thinking this has something to do with some weird Firefox configuration thing.
What might be the problem? What can I do to investigate what's going wrong?
edit — Uninstalled/reinstalled Firefox, no effect. Still can't find another application on the system that doesn't understand the ALT key.