Have you tried Safe Mode? Support: Safe Mode If Safe Mode helps, then you need to disable Add-On

Have you tried Safe Mode?

Support: Safe Mode

If Safe Mode helps, then you need to disable Add-Ons/Plugins to find out which on is messing things up.

There were reports of this some time ago, back in FF3&4, nothing certain was said to reslove the problem. Some people noticed; Firebug & Delicious add-on were causing the issue, others noted that disabling the "Smart Location Bar" helpw.

I just disabled FF Sync to fix the issue when I had troubles with the Address Bar randomly not working. but that was much later than FF4