I have this problem too on Firefox 4.0 Beta 3

ask toolbar add-in disable it problem goes away

See also: Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

It is not an extension that causes the problem, it is the extension maganer that crashes. Cannot open The add-ons window at all: Firefox crashes completely.

stammeshaus, try running Firefox in safe mode, and use it to disable all extensions. The error message is caused by an incompatible add-on. I have seen reports of it with a few add-ons, this is due to changes in Firefox which made the add-ons no longer work.

The extension manager can crash with a possible cause of corrupted files extensions.* in the Firefox Profile Folder

You can delete the files extensions.* (extensions.rdf, extensions.cache, extensions.ini, extensions.sqlite) and compatibility.ini in the Firefox profile folder to reset the extensions registry. New files will be created when required.

See "Corrupt extension files": http://kb.mozillazine.org/Unable_to_install_themes_or_extensions

If you see disabled extensions that are not compatible on the next start in "Tools > Add-ons > Extensions" then click the "Find Updates" button to do a compatibility check.

After reading a few posts, I looked thru my extensions. I disabled Ask.com tool bar and the problem went away.

Just go to tools then add-ons you will find extensions , open and there you will find Ask.com , just delete it , but this is only if you already deleted the ask tool bar from control panel and you didn't delete it from the extension add-ons , that;s why it makes this error if you delete the ask.com tool bar and you left this without deleting it. Cheers guys

I have this problem too on Firefox 4.0

Uninstall the mozilla including personal info and re-install. problem is solved.

I disabled the Avery toolbar extension and no longer have this problem.  :)

Removing the ask toolbar worked for me as well.. I don't get why big companies like ask release things that make browsers bug out?!?!