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

Thank you so much! I uninstalled the GOM player + Ask Toolbar and I can open new tabs again. Again, thanks!

Lee

You're welcome.

Please click the Helpful button next to the answer that solved your Firefox support issue, when you are logged in, so this thread gets marked as Solved to help other users who may have this same problem.

I already marked it as helpful, but when I click on the 'Solved it' button it says "Access Denied"...?

Looks that marking as solved is currently disabled.
I've seen other threads with similar problems marking as "Solved it" not working