Or if that frightens you, and you want Google as your homepage (or one of them):
Open all the sites you want as your "start pages" in separate tabs - mine has five, one of which is Google.co.uk
Click Tools - Options - Main
Under "Startup" click "Use current pages".
Then, when you start Firefox, it will open PPRuNe, Google, your stockbroker, the Aviation Weather, etc all at once.