Firefox 4.0 introduced OpenGL acceleration support for WebGL and HTML5 content, but the support was bound to Windows and Mac OS X. The developers acknowledged that the GPU Linux drivers were in bad shape, but just not the open-source ones -- the proprietary drivers failed too. The only driver not breaking miserably in most situations was the proprietary NVIDIA driver.
As mentioned in May, with Mozilla Firefox 6.0 are the Linux drivers finally beginning to be white-listed en masse. Those running the proprietary NVIDIA driver (v257.21+), the AMD Catalyst proprietary driver with OpenGL 3.0+ support, or Mesa classic DRI drivers (v7.10+) will now have full GPU acceleration support exposed for HTML5 and WebGL content. Note that it's "classic" drivers in Mesa, but not Gallium3D. The Gallium3D drivers are still being black-listed by Firefox until the 7.0 release.
Look for Mozilla Firefox 6 to be officially released in the next two days, or you can grab it already from an FTP mirror.