If you don't make IE or Firefox the default you can use both as you want.
If you make one the default then when you open links in other programs or documents or email the default one will be used.

to change the home page just put the MSN link into the location bar - hit enter to go to it
then go up to Tools / Click Options then click on the Main tab
then click on the use current page

close and reopen to see if MSN is set as home page for you.

or you can just copy and paste the MSN link right into home page area and click OK to save it