May already have known about it, and I'm sure I discovered it by accident, but in Firefox, if you type the following in to Tools>Options>>General>> Homepage: Code: www.google.com|www.hardwareforums.com|www.bbc.co.uk|www.play.com (for example) Firefox will automatically load each of the sites into separate tabs. Sort of handy, I suppose..
You can also just type them all in tabs. Then you can go to Tools -> Options -> General, and there is a button for "Use Current Pages."