I've had some minor scrolling issues with iceweasel-4 which mostly did not draw my attention until I came across this page : http://tech.shantanugoel.com/2009/07/07/virtualbox-high-cpu-usage-problem-solved.html. Scrolling was very choppy.....
Some background : system : Debian squeeze amd64 gnome, nvidia 270.41.19 ( nvidia.com ).
apt-cache policy iceweasel
I've tried different gtk-styles, different window-managers, disabled all plugins and extensions, created a new profile.....no improvement....
Some others things I've noticed : running iceweasel-4 in virtualbox ( minimal gnome installation Sid ) did not have these scrolling issues.....( so is nvidia to be blamed ? ).
Out of curiosity I uninstalled the nvidiadriver and used nouveau : scrolling issues were gone...
( that was an one-time experiment ).
I tried just one more step : Added deb http://hadret.rootnode.net/debian/ unstable main to sources.list and upgraded libcairo2 from version 1.10.2-1.1~bpo60+1 ( squeeze-backports ) to version 1.10.2-6.1. ( http://hadret.rootnode.net/ )
Problem was mostly solved, but I still have some trouble on certain pages. An example this page : http://www.groupon.nl/.
Scrolling gets sticky past the middle of the page....
Next tweak I found out ( scrolling issues disappear ) was to start iceweasel with MOZ_ACCELERATED=1 /usr/bin/iceweasel
Or you can go to the about:config page and set layers.acceleration.force-enabled to true.
If you go to Help > Troubleshooting Information you will see at the bottom of the page :
GPU Accelerated Windows 1/1 OpenGL
With certain themes this tweak will make the tab animation choppy, but you can switch it off by going to the about:config page and set browser.tabs.animate to false.
Get a picasa-like scrolling with : Yet Another Smooth Scrolling.