Firefox should save energy
I really like the idea presented by someone else that "Firefox should sleep hidden pages" but for another serious reason.
If my computer's fans are running at full speed and the CPU is running 100% it means it's drawing near maximum power. These days when there is so much attention to IT departments trying to save money on energy costs with more power efficient computers, I think people also need to consider all those desktop computers everywhere chewing up more energy than they need to.
It is not always practical to sleep hidden pages because they might be doing something useful, like playing music from a radio station. However, I think it should be possible to configure Firefox to go into power-saving mode after some set period where it actually does sleep all scripts and other browser activity. When you come back to your computer it should be apparent in the UI that Firefox is in power-saving mode and wake it up. Maybe this could be made automatic somehow when you move the mouse or press a key on the keyboard.
As far as sleeping hidden pages go, one solution would be to automatically sleep hidden pages, but implement a control (like a check-box) somewhere to indicate a particular page should not sleep (i.e. because it's playing music).