You shouldn't be assigning the XSL file type to the browser. All that does is tell Windows to use the browser to open the XSL file if you double-click on it.
Instead, open IE, and on Internet Options -> Programs tab, click on the "Make Default" button to switch your default browser to IE. (Or, open Internet Options in your control panel and do the same thing.)
Start up Firefox and go to Tools->Options->Main to switch the default browser back to Firefox.
Sinc