I have exactly the same phenomenon here on my HP POSwhatever work laptop, in both FB and IE... But then, my right "Alt" key isn't an "Alt" key, exactly -- it's clearly marked "Alt Gr".
My guess is, either (possibly but not very probably IMO) your Big Blue (or Blue Dragon?) lapper has a similar keyboard, only marked "Alt" on both sides, or (more probable AFAICS) some keyboard driver or whatever in your install makes it behave as if it did.
K-ubuntu, you say? As in, Ubuntu with KDE -- that very-German-influenced, if I understood correctly, desktop environment?
That's my prime suspect, anyway.
HTH!