webgl conformance tests crash radeon driver, but not fglrx

There seems to be a bug in the open source radeon driver.
https://www.khronos.org/webgl/wiki/Testing/Conformance
https://www.khronos.org/registry/webgl/sdk/tests/webgl-conformance-tests.html
One of the tests crashes firefox (but not X). A great many pass, many fail.
OS radeon driver.
No crash with closed-source fglrx driver
AMD R7 260X graphics card.

Obviously this *might* be related to https://bugs.launchpad.net/linuxmint/+bug/1419099

1. Mint 17
2. Use the open source radeon driver. Visit https://www.khronos.org/registry/webgl/sdk/tests/webgl-conformance-tests.html with firefox (it was about version 33, but the latest probably still has the problem).
3. firefox crashed
4. firefox would not crash
5. Always

It might be a bug with firefox, but given that fglrx works, it is probably a bug with the radeon driver.