You can change default browser settings to Firefox by selecting the following menu:
Tools > Options > Main
At the bottom of the menu you'll see a section marked "System Defaults". To set the default browser to Firefox click the Check Now button.If your computer does not have Firefox as the default browser it will give you the opportunity of changing the default browser to Firefox.
Change the check box of "Always check to see if Firefox is the default browser on start-up", will tell you next time Firefox isn't the default browser.