Shot in the dark here, but do you have Firefox set as your default browser?
Unless UO is hard-coded to use IE, theoretically you would think it would just use the default browser.
If you go to Tools -> Options -> Advanced, and look on the General tab in Firefox, you should see a checkbox so that Firefox will check to see if it is the default browser every time you start up. If you check that box, or hit check now, and say yes, it should make it the default browser, and you can check to see if that solves your problem.
Hope this works.