Could be a malfunctioning script, perhaps? I think the first thing I'd probably do is start the browser with no add-ons (Help > Restart with Add-ons disabled), then if that cures the issue, enable them one by one and see if any of them are causing the problem. You can also go to http://www.mozilla.org/en-US/plugincheck/ to check if all of your plugins are up to date. Failing that, reinstalling the browser is probably the next thing to try.
"Add-ons disabled" mode probably disables everything, including plugins and extensions. So, maybe it's a plugin or an extension causing it? Do you get the option to disable them in Add-ons Manager?