Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

In Firefox 4 you can use one of these to start in Safe mode:

• Help > Restart with Add-ons Disabled
• Hold down the Shift key while double clicking the Firefox desktop shortcut (Windows)

I was able to narrow down the add-ons Shareaholic and CoolIris Previews as the culprits in my case. If you have either of these, try disabling them. It solved my problem. My guess is not all add-on publishers have their stuff working in FF 4. In any case using the safe mode and disabling all add-ons, then going through them one-by-one goes a long way to fixing this issue...annoy though it may be.