Uninstall the Ask toolbar and it should work again. There is a compatibility issue with the Ask toolbar and Firefox that prevents new tabs from being opened.

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

I also had this problem, and solved it by uninstalling the Avery Toolbar extension

Thanks TonyE I uninstalled the ASK toolbar and my tabs are operating fine now... Kris