MacBook Pro (15-inch, Early 2011)
Processor 2 GHz Intel Core i7
Memory 8 GB 1333 MHz DDR3
Graphics AMD Radeon HD 6490M 256 MB
Software OS X 10.8 (12A269)
Darwin Kernel Version 12.0.0: Sun Jun 24 23:00:16 PDT 2012; root:xnu-2050.7.9~1/RELEASE_X86_64 x86_64
In both: Google Chrome Version 21.0.1180.77 beta
Mozilla Firefox 14.0.1
Edits: updated formatting and newlines. I don't see an option to enable high performance graphics in Mountain Lion, but I tried disabling auto-switching of graphics and it has the same issue.