Like what?
When using IE I was forced to run Ad Aware everyday and it would always find 20 to 30 spyware programs every time!
Then there is tabbed browsing which works great. The feature I like the most is you can highlight any word or phrase on any webpage and have a Google search done on it, which opens up in a new tab. I love that!
IE is dead, long live Firefox!!
Tabs, RSS feeds (live bookmarks, great for FR), easy extensibility, quality popup blocking (SP2's isn't that good), better privacy and security tools, view selection source (also excellent for FR posting), and for the developer, excellent code debugging. Google searches are also integrated (no Google toolbar necessary) including selecting some text and clicking to open a Google search for that in a new tab.
With a couple quick extensions added, you get intelligent ad blocking and a whole bunch of other useful stuff. I use dictionary search, plain text links and GMail notifier.
All of the above has sped up my web browsing a lot.
One is only useful because we're in an IE world. Some sites, and lots of pages at Microsoft, either refuse your visit or give a warning if you're running Firefox. I can quickly set my user agent to IE to view these sites. All of us doing this has lowered the usage statistics for Firefox and added to the IE numbers.
And if you know JavaScript it's easy to write your own extensions.