Multiple Browser Start-up Pages
Because Internet Explorer (Version 7.0) and Firefox use tabs, it's possible to have more than one web page as your start-up page. To do this, open your favorite pages on different tabs. If you're using Firefox, go to the Tools menu and select Options. Click the Main tab. In the Startup area, click Use Current Page, then click OK.
If you're using Internet Explorer, go to the Tools menu and select Internet Options. Click the General tab. Click the Use Current button, then click Apply, then OK.
Although Safari also uses tabs, I couldn't get this method work with it.