So Firefox is also expensive to run in terms of energy consumption. No wonder the fans on my MacBook Pro always sound like a jet engine whenever I have several tabs open. Seriously! Disclaimer: I dual boot (camp) windows 7 on my mac.
I doubt the cpu load of session restore is responsible for the fans kicking in...
Well I must say, I certainly changed my settings to 45 minutes and OMG! fans haven't spin up loudly since. Wow!