I am pretty sure its on a per browser, not global:
1. go to http://us.blizzard.com/en-us/
- must be a site with a large enough flash frame or the flash settings will get "stuck" and require you to reboot your browser.
2. Right click in the middle, select "settings"
3. Select the leftmost tab (tabs are at the bottom)
4. uncheck the GPU acceleration option.
You might have to restart the browser (or just tab in chrome) a few time as its prone to freezing / crashing / doing weird stuff.
Beyond being very buggy (I hate flash with a passion, it burns like a thousand suns!), flash GPU acceleration also causes my GPU to get very loud, very noisy, and waste my precious (expensive) electricity.
IE9 GPU acceleration doesn't do that BTW, just stupid adobe failash