I don't believe Firefox can affect the functionality of your USB2 ports in WIN 8.1 though it may appear so on the surface. If it did, there would be a number of complaints on the net about the issue? I've had no problems with USB2 at all with v30. Does Device Manager(devmgmt.msc) indicate a yellow exclamation with Firefox v30 installed under Universal Serial Bus controllers?
What do you use your USB2 ports for with Firefox installed? If its printing from the browser, then that could be caused by a Firefox profile issue. Either corrupt or has some type of PUP? Did you create a new(clean) profile in Firefox as a check?
Another issue is to check to make sure your AV software and Firewall is not blocking Firefox access to USB2 ports for a security reason?
In the meantime, here is the Mozilla official archive link Firefox v29.01 for
EN-US or
EN-GB if you don't ready have them.