I run firefox on an ubuntu 9.04 computer. I monitor power consumption with powertop. Firefox is the worst culprit when it comes to battery waste. Sometimes, out of the blue, it keeps running at >90% CPU utilization. At least that is easy to detect because the computer gets hot.
But more often than not, Firefox keeps the CPU busy in state C0, and even if its CPU usage is small, the CPU just can't go into power savings mode. Firefox can account for 3-10 watts of usage on a regular session.
The problem is, even if I close all the windows/tabs and leave only a static document in the only window left, firefox keeps using the CPU. The only solution is to kill it and restart it. It is frustrating.
Whenever I want maximum battery life, Firefox is killed first, followed by the wireless card (using the physical switch). By doing these two things I can expand battery life by 50% (assuming Firefox is not misbehaving my laptop uses ~ 15 watts, 10 without firefox and wireless, and screen at 30% brightness).