The Ask Toolbar is causing that in the Firefox 3.6.+ 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 would never have worked it out!

I have Firefox v3.6.3. I do not have the "Ask Toolbar" but I did have "Search Toolbar" that came in without me requesting it. I uninstalled this Search Toolbar Add-On. I can now click the + and it now works again (i.e., a new tab is opened).