I suspect that Firefox is relying on modern-ish GPU features to make things "fast", but Mesa is straining the CPU to emulate those features, which are missing from your older GPU. (Software going "I'll use the GPU, that must make things faster!" is what finally pushed me away from using an x60 as my daily driver).
it might be my ancient hardware, but this is rather CPU intensive it seems.
But the effect is fun, and cool non the less!
(on Linux, Xorg + Frirefox)