WebGL in Firefox (4.0b1) is very slow on r300g, it seems to be using an excessive amount of CPU, this is not the case in Chrome, where things seem to be working very well. 

I'm not really sure if this is a bug in the driver, or in Firefox, but on i965 performance seems to be the same in both browsers.

This problem is very noticeable in demos which measure framerate, such as:
http://cs.helsinki.fi/u/ilmarihe/demo/demo.html
http://cs.helsinki.fi/u/ilmarihe/demo2/paint.html

Another good test is an interactive demo like the teapot, where lag is noticable:
https://cvs.khronos.org/svn/repos/registry/trunk/public/webgl/sdk/demos/google/shiny-teapot/index.html

WebGL requires support for pbuffers (for both Firefox and Chrome) so Xserver 1.9.0 is required. For more general tips on getting webgl running see: http://learningwebgl.com/blog/?p=11

System environment:
-- system architecture: 32-bit
-- Linux distribution: Debian unstable
-- GPU: RV570
-- Model: Asus EAX1950Pro 256MB
-- Display connector: DVI

-- xf86-video-ati: 6.13.1
-- xserver: 1.8.99.904 (1.9.0 RC 4)
-- mesa: f8d81c31cee30821da3aab331a57f484f6a07a5d
-- drm: 6ea2bda5f5ec8f27359760ce580fdad3df0464df
-- kernel: 2.6.35-rc5