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

That issue can be caused by an AVG extension (AVG Safe Search).

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

just disabled agv and it now works fine, thank you so much cor-el :D

I have this problem as well, but when I go to my extensions nothing shows up! What can I do?

Problem solved. For those of you having the same issue, just go back to a previous version of FF and you'll be able to see your extensions. Then just disable the AVG safe search. Hope this helps!

It is possible that there is a problem with the file(s) that store the extensions registry.

Delete the files extensions.* (e.g. extensions.sqlite, extensions.ini, extensions.cache) and compatibility.ini in the Firefox profile folder to reset the extensions registry.

• https://support.mozilla.org/kb/Profiles

New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
• https://support.mozilla.org/kb/Unable+to+install+add-ons

If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left side of the Search Bar (or click the "Find Updates" button in older Firefox versions) to do a compatibility check or see if there is a compatibility update available.

You may have to reset the preferences as well.

• http://kb.mozillazine.org/Resetting_preferences
• https://support.mozilla.org/kb/Resetting+preferences