your useragent (the information the browser sends to websites to identify its version) go overwritten probably by some sort of toolbar/adware. please refer to this article in order to reset the user-agent settings in firefox: How to reset the default user agent on Firefox

I agree with madperson that it could easily be due to your not default Firefox useragent which is "Mozilla/5.0 (Windows NT 6.1; rv:19.0) Gecko/20100101 (EB0A856D-C9DC-6065-31F7-8E5E0910164B) Firefox/19.0" based on your "More system details" link on right of your post.