I'm using Mesa 8.1-devel (git-e1a173f) with Linux 3.4.0-rc2 and Mobility Radeon HD 5470 and Firefox nightly 20120419030644
If I run the WebGL conformance tests from here:
one of the failing tests is
failed: pixel != 255 && pixel != 0 should be true. Was false.
Fixed by firefox
works with mesa 8.0.3, mesa master
and firefox nightly