New Tab function has just stopped working. New window works. Any ideas?

I've been using Firefox about 3 years with no problems. As of today, the Open New Tab function does not work, and I can't find any place where that is user controlled. I can still open a new window, and everything else appears to function normally, but can't start a New Tab. A new tab appears if I open a link elsewhere, but won't open in response to a manual request.