Having multiple tabbed start pages in Firefox
One thing that I love about using Firefox is being able to set an array of pipe-separated URLs to be used as independent start pages upon startup, via that browser's multi-tabbed UI.
I'm the type of guy that prefers using web-based apps, so I typically need to have multiple pages load every time I launch my browser. I typically have running at any given time my blogging app, GMail, a mapping service, a web-based RSS reader, Flickr, some tagging services, and some admin utilities I've built. Really intuitive feature that I hope MSIE 7 has, too.