yes go to the mozilla firefox extensions page
there is an extension called Hotmail Tabs
I registered for a new hotmail addy (.co.uk) yesterday and it worked for that so go download that 
Ignore that, that is for hotmail tabs
I also have this problem, no it doesn't like to open the tabs in firefox when it is your default browser (not sure about mozilla though)
I have no idea why, can ii use regedit to change it somehow?