Confirming, scrolling Launchpad bug reports in Firefox is sluggish, both with drivers 195.36.24 and 256.35. Seems these pages trigger some type of unfortunate rendering interaction between Firefox and Compiz|Xorg|nvidia. Xorg consumes much CPU during sluggishness (contrary to firefox or compiz processes). Ubuntu 10.04 x86-64, 8 GB RAM, Intel Core i7 620M, NVIDIA NVS 3100M 512MB, Compiz. No special nvidia-driver options in xorg.conf.
For example, the top few pages of this bug report trigger slow scrolling:
.. when scrolling a whole page at a time (e.g. pressing space), or using scrollbar with mouse.
When you reach the comments section, scrolling becomes better, which indicates that some page elements on the top of the bug report trigger the performance problem.