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

Thanks, TonyE, but I've never installed the Ask Toolbar. Double-checked in Control Panel to make sure they didn't slip it in on me.

To amplify my original post, I've tried clicking the plus button at the right of the right-most tab, right-clicking that button and getting a new tab choice in the drop-down menu, and the same with File/New Tab.

Firefox opens a new tab when I go to another website. It's just that I can't open a blank tab manually. I've selected all the Tab choices in Tools/Options. Still no new manual tabs.

Your user agent string is "Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.16) Gecko/20110319 AskTbAD3/3.11.3.15590 Firefox/3.6.16"

The "AskTbAD3/3.11.3.15590" part shows that the Ask toolbar is installed. check teh add-ons manager to see if it is listed there.

The ASK Toolbar didn't show in the Add-Ons manager. But the Avery toolbar (which I never use) WAS there. I uninstalled it, restarted Firefox, and the problem was solved.

Many thanks.