I have spent this day looking for a solution to my Firefox's slow scrolling(and browsing) on Ubuntu 8.10 . It was not just Firefox, Ubuntu GUI was also slow sometimes.  So I understood that the problem was not in Firefox only. I have tried to install the latest ATI graphic drivers for my X1950, but that didn't happen.  When I changed my CPU frequency from "Ondemand" to "Performance" my computer was super fast again (with visual effects set to extra). Changed it back to "Ondemand", Ubuntu was slow again. So I selected the "Performance" option and went back to Firefox to check how it works. Surprisingly it didn't lag any more. Scrolling was smooth, scriptolous websites like Digg loaded fast again and i am happy i found a solution ;)

To change the CPU frequency, I added the CPU Frequency Scaling Monitor element to my panel (taskbar) and left clicked it choosing the "Performance" option.