I've had gfxCardStatus on my machine for over a year. Changing the GPU has no effect.
I also tried using a completely new Firefox profile--didn't solve the problem.
The behavior is limited to Firefox and seems entirely isolated to my user account.
The only other culprit I can think of (I'm grasping at straws
here) is SIMBL, which I've used for a few years to get back the look of a transparent launch pad.
Other than that, I'm ready to give up and create a new
user account and move "non-controversial" stuff over
there. (Been using Safari lately, but don't like its UI very much).