Juz add a pipe [|] symbol in between the urls you are gong to type. Yeah, dont forget the space in between..
so if you want google.com and yahoo.com in multiple tabs,
then go ahead and write
www.google.com | www.yahoo.com
in your firefox tools > Options > Main > Homepage
The original entry can be found in
Ask The Admin: Can I Have Multiple Tabs for My Home Page In Firefox?