Strange advice from Diz.
Diz, doing it that way will simply remove the associations with IE, an will leave no default browser set, causing problems when a link is clicked in any other application.
Puzzeld,
in firefox click tools > options, click on the advanced heading at the top and towards the bottom is a section about default browser, put a tick in always check if firefox is the default browser and click check now next to it, click OK when it asks if you want to make it the default browser.