UserPrefs: webgl.force-enabled = true $ glxinfo | egrep vendor\|renderer\|version server glx vendor string: SGI server glx version string: 1.4 client glx vendor string: Mesa Project and SGI client glx version string: 1.4 GLX version: 1.4 OpenGL vendor string: Mesa Project OpenGL renderer string: Software Rasterizer OpenGL version string: 2.1 Mesa 7.10.2 OpenGL shading language version string: 1.20 Firefox 5 does not crash and just informs the user that it can't create a WebGL context.

Can you please go to about:support and paste what the Graphics section says. The fact that it's blacklisted and you had to set webgl.force-enabled, while this setup is normally whitelisted, suggests that maybe the glxtest process correctly caught the crash.

Adapter Description: GLXtest process failed (exited with status 1): X error occurred in GLX probe, error_code=9, request_code=55, minor_code=0 WebGL Renderer: Blocked for your graphics card because of unresolved driver issues. GPU Accelerated Windows: 0/1. Blocked for your graphics card because of unresolved driver issues.

Yeaaaaaaaahhhhhhhh! \o/ So the glxtest process successfully caught this crash. Firefox itself is protected from it unless one force-enables. If you would like to get this further debugged in Mesa itself, feel free to file a bug at https://bugs.freedesktop.org

CC'ing Karl who's been looking at similar bugs recently. See if this is a dupe?

Mesa bugs already filed and linked from bug 589546. Bug 589546 comment 52 explains why this shows up as X_FreeGC: BadGC.