Blame the Add-Ons in Firefox, Too
(from PCMAG.com article)

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02.13.09
Blame the Add-Ins in Firefox, Too
by Neil J. Rubenking

Q: I am using Firefox 3.0.5. While I'm browsing, for no reason, a box sometimes pops up saying "URL not valid and cannot be loaded." I remove it by clicking OK, but I find this annoying, especially since the frequency is very high. This happens only in Firefox. Is there a short easy answer for a rather computer-illiterate old man? - Brian Foord

A: When a strange problem crops up in Internet Explorer I blame the add-ons, but Firefox, as well, isn't immune from add-on-induced bugs. Start Firefox in Safe Mode to see if an add-on is causing your particular problem. How? First, close all Firefox windows. Then in the Start menu's Run dialog enter firefox.exe -safe-mode. If for any reason that doesn't work, replace firefox.exe with the full path of the program, probably "C:\Program Files\Mozilla Firefox\firefox.exe"—including the quote marks.

Check the box to Disable all add-ons and click the button to Continue in Safe Mode. Problem gone? Okay, exit Safe Mode, restart normally, and re-enable the add-ons one at a time until you find the one that's the source of the trouble. Then either leave that one disabled or seek an update that fixes the problem.