I see the same, but top shows the spike in Xorg, not in Firefox.
Disabling the screen (and therefore the shared and high) stylesheet makes things snappy again.
Dell Precision 670 (dual Xeon 3.0) Ubuntu 8.10 2.6.24-19-generic Firefox 3.0.1 nVidia NV45GL also using the restricted "nvidia" driver
I see the same, but top shows the spike in Xorg, not in Firefox.
Disabling the screen (and therefore the shared and high) stylesheet makes things snappy again.
Dell Precision 670 (dual Xeon 3.0)
Ubuntu 8.10
2.6.24-19-generic
Firefox 3.0.1
nVidia NV45GL also using the restricted "nvidia" driver