I recently received the following comment and I thought it would be a good idea to go over some reasons why a web browser might get sluggish.
WTF, did you test this page? It is so freakin slow to scroll in FF3 - your layout sucks 100%, seriously.
The most common reason is that you have may have too many tabs open. Sometimes it could be a buggy extension you installed; While most extensions are well done, there are a few that are really not ready to be used. One of your tabs that has a lot of JavaScript or Flash going on. Sometimes if you have a Quicktime plugin loaded in another tab that can bring your browser to a crawl.
There have been times that I’ve left Google Reader open all day at work in a tab and sometimes it will just end up making my browser so slow I have to relaunch it. Although it hasn’t happened that often with Firefox 3.