"new activity" indicator on pinned tabs uses a LOT of CPU.
My laptop was churning through battery like crazy. After plugging it in, the fan continued to run high, and I was trying to figure out what was using all the CPU. I checked the Activity Monitor (on Mac OS X) and it said Firefox was using all the CPU. I then noticed that two pinned tabs had the red pulsing background to indicate activity on them. Switching to those tabs to clear the indicator immediately stopped the heavy CPU usage.