Turns out this was/is a problem with a new version of xserver-xorg-video-intel.
The problem disappears when using vesa driver or downgrading to an older
version of xserver-xorg-video-intel.
For anyone interested: I have an Intel X3100 (GM965) and
xserver-xorg-video-intel Version 2:2.2.0-1 does no
I have the same problem. (with Sidux (Debian Sid Distro), my graphics
card is an Intel X3100 (Intel GM965) using xserver-xorg-intel and mesa)
Xorg cpu usage skyrockets while firefox is rendering a page, or while entering
text in the addressbar for example.
Maybe this is a gtk problem not a firef
