Sounds like a conflict with Addons, which I experienced a few versions ago. '''Try the Firefox SafeMode''' to see how it works there. <br /> ''A troubleshooting mode, which disables most Add-ons.'' <br /> [https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode#w_safe-mode-window_2 Troubleshoot Firefox issues using Firefox SafeMode] When in Safe Mode... <br /> * The status of plug-ins is not affected. * Custom preferences are not affected. * All extensions are disabled. * The default theme is used, without a persona. * userChrome.css and userContent.css are ignored. * The default toolbar layout is used. * The JIT Javascript compiler is disabled. * Hardware acceleration is disabled. * You can open the Firefox 15.0+ SafeMode by holding the '''Shft''' key when you use the Firefox desktop or Start menu shortcut. * Or use the Help menu item, click on '''Restart with Add-ons Disabled...''' while Firefox is running. ''To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shft key) to open it again.'' '''''If it is good in the Firefox SafeMode''''', your problem is probably caused by an extension, and you need to figure out which one. <br /> http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes ''When you figure out what is causing that, please let us know. It might help other user's who have that problem.''

Thanks to the-edmeister for the help. The problem, it seems, is caused by the Google Toolbar 7.1.20110316W add-on. I've decided that the problem is worth living with to be able to continue to use this now normally incompatible add-on.