The Ask Toolbar is causing that in the Firefox 3.6.13+ versions. Uninstall that extension.

There are a couple of places to check for the Ask toolbar:

• Check the Windows Control panel for the Ask Toolbar - http://about.ask.com/apn/toolbar/docs/default/faq/en/ff/index.html#na4
• Also check your list of extensions, you may be able to uninstall it from there - https://support.mozilla.com/kb/Uninstalling+add-ons

possibly [make that possibly]

• http://kb.mozillazine.org/Corrupt_localstore.rdf

If MozillaZine is still down when you read this, try Google's cache copy

Didn't see Ed's answer, his answer is probably the one, if you have that extension.