I cannot click on tabs in any website, do you know why? I can do it when using Internet Explorer but not firefox.

I have used Mozilla Firefox for years, but all of a sudden I cannot put my curser over a tabs on any website and click on it. What I mean is the top of the websites have words you can click on to go to other pages or in the email system it has tab buttons that allow you look at your inbox or close or save draft, it doesn't allow the curser to click on it. I can go to the tool bar put my curser in that and use the tab button on the key board to tab over to each word or link and then click enter and it will then go to that next page.