[jo_]

There are enough factors to make this answer little more than speculation. Please take it with a grain of salt and not as evidence for a course of action.

In theory, the Javascript engine is significantly faster in Chromium than in Firefox[1]. I don't think your system is falling back to CPU rendering (you would notice), but the overhead associated with WebGL is probably higher in Firefox. Additionally, since Chrome runs each tab in its own process, there is perhaps less overhead when switching task context for GL rendering.

[1] According to the April 2013 numbers on http://arewefastyet.com, they're pretty much on par. I can't shake the idea that Google's V8 engine is faster, but I can't prove it, either.