Firefox Loud Fans But No High CPU Usage

After updating Firefox to 32.0 the laptop fans spun up even though Task Manager did not show a busy CPU. The machine sounded as if it were compiling a kernel. It was rendering web pages with hardware acceleration and the GPU was doing the work the CPU meter never shows.

If the same thing happens now: open Firefox settings, search for performance, and try turning off hardware acceleration. Restart the browser. If the fans drop, you have your answer. Drivers and a later Firefox release may fix it properly; the checkbox is the quick test.