This might have been around since Mozilla added tabbed browsing, but Firefox has the ability to have multiple pages start as your home page.
1) Start Firefox and setup the tabs to the sites you want
2) Open the Preferences page:
Edit -> Preferences -> General.
3) Click the “User Current Pages” button.
That’s it. Notice that it simply puts in a PIPE (|) character between each URL.
Now this works well at startup, but not so much when you press the Home button which promptly opens two tabs, and if you’re currently in a tab the first homepage appears there then a new tab is opened the second homepage.