Uninstalling Firefox and then re-installing it from www.getfirefox.com would be a good idea.
This link http://www.mozilla.com/en-US/firefox/all-beta.html
will take you to the latest Firefox 3.5b4 beta.
You could also try using Google Chrome, www.google.com/chrome, another excellent and fast browser, while Opera 9.6 www.opera.com is another great alternative.
Cheers
Alex.
