I recently had a problem with my GPU not downclocking properly when idle (i.e. in 2D mode, with no 3D applications running). Some browsers use hardware acceleration and offload rendering to the GPU, but this was happening even with no browsers running.
nVidia's HD Audio Driver that gets installed alongside the video driver is known to cause this, but I'd already gotten rid fo that, so it had to be something else; turns out the culprit was Thunderbird. So if you find yourself with the same problem, check to see if your GPU will downclock if you exit Thunderbird; if so, you may want to disable hardware acceleration:
- Open the equivalent of about:config, by going to Tools → Options → Advanced → General and clicking on "Config Editor...".
- Set gfx.direct2d.disabled to true.
- Set layers.acceleration.disabled to true.
You may have to restart Thunderbird for these changes to have an effect.
After I did this, my GPU downclocked again (and as a side effect, its temperature fell by a full 10°C as well).