Can you connect with IE or Chrome?
I agree with Ian (assuming that is the add-on you speak of). Although it appears there is only 1 review, I would be discouraged by an add-on for one browser affecting another browser. That sounds like sloppy programming to me but 1 review is not a trend.
Still, I have found that many problems with browsers are not the fault of the browser, but wayward add-ons not working properly, or conflicting with other add-ons. So I tend to use as few add-ons as possible and keep my browser as a browser, instead of asking it to do other things it is not.
I recommend you uninstall that add-on and see if you can connect. If you can, dump the add-on permanently. If you still can't, but can with another browser, you may need to reset or reinstall FF. If you can't with any browser, then you have a security setting somewhere messing with you.