Let me quickly mention an important point that I missed in my previous review of the about:tab for Firefox 3.1.
If any of your "most frequently visited" website has an RSS feed, Firefox will automatically display the latest news items from that feed when you open a new tab in Firefox 3.1 / 3.5. (see screenshot).
You can also customize the number of stories that are displayed per feed and Firefox will remember the settings across all browser sessions.
I am not sure how frequently is Firefox checking for new content but this looks like a pretty useful feature.