This issue is detailed here:
The proof-of-concept consists in loading a cross-domain image as a WebGL
texture, and rendering each pixel using a shader program that takes an
amount of time proportional to the R, G, B values of pixels. By timing
this, they get an approximation of the image.
RHEL-4/5/6 ships firefox 3.6.x, which does not have support for WebGL.
Not Vulnerable. This issue did not affect the version of Firefox as shipped with Red Hat Enterprise Linux 4, 5 or 6.