I came across this as I visited ftp://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/ with the current patch from bug 294800 applied, which adds a border radius to the FTP dir listing. The page is about 12,000 px high and the performance is even worse than in the textcase (e.g. autoscroll hardly works at all), probably because there's real content. If I remove that single border-radius property, scrolling works perfectly smooth.

Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9a7pre) Gecko/2007072101 Minefield/3.0a7pre

non-issue with Firefox 2

Interestingly, I don't see anything of the bad behaviour if Ubuntu's "Desktop Effects" are disabled. Since they are off by default, I guess this isn't a blocker.

Depends... What does "Desktop Effects" actually do?

Currently available are "Windows Wobble when moved" and "Workspace on a Cube", and it uses Compiz. I just reenabled the effects but can't reproduce the bug anymore. Dunno what happend in the meantime. Maybe an update of the Nvidia driver or the Cairo upgrade (assuming my build wasn't the latest one when I wrote comment 4, which could be the case, given that I don't access that computer daily.)

Desktop Effects uses Xgl, and runs a compositing window manager (compiz). The RENDER implementation there is questionable at best, I think.