This page demonstrates a bug in Firefox 0.8. This bug is filed as Mozilla Bug #163975. I have observed it on my Win2K system, but it is presumably present in the Linux version as well.
Another bug (Mozilla bug #215090) shows a user reporting similar behavior in Linux when scrolling past the large image at http://www.nycwireless.net/poe/. I too witness that slowness on my computer (though it does not interfere with keyboard events in Win2K as that bug describes in Linux -- same problem though). Bug #238423 also describes this behavior.
I can visually feel the sluggishness of scrolling the test case #2 or #3, and if you view the Task Manager, you can see CPU usage in Firefox go up to 100%. There is some problem with scrolling resized images, even exactly 50%, which should be easy.
The image used in this test is a 500x550 JPEG.