Dude. I know a lot of you use Firefox, and I applaud you for this. RC2 is out, just in case you're wondering.
One thing I really like is RSS feeds. They allow you to keep up to date on sites and blogs and such.
There are quite a few ways to get RSS syndication in Firefox/Thunderbird/Mozilla:
1. Use the preinstalled stuff (Firefox/Thunderbird only). - Firefox has livemarks, which are kinda nifty. The detection isn't great, though, and there's no way to add livemarks using the URL of the feed, which is sad. It's still a pretty cool idea to have a feed inside a bookmark. I personally haven't used it much, but it seems pretty good.
Thunderbird's integration is a little better. You have to dig up the URLs yourself, but it's nice. It acts like another mail account, which fits in with the program. Plus, it's pretty easy to use.
2. Forumzilla (Mozilla and Thunderbird only) - Basically adds Thunderbird's RSS functionality to Mozilla. It was an extension that was probably checked into Thunderbird's code and modified a bit. I just installed it today. w00t. It's a little on the awkward side, but overall slightly more powerful than Thunderbird's built in version.
3. Sage (Firefox only) - One of my favorites. I've been following it since the RSS Reader panel days. It uses bookmarks brilliantly as the subscribed feeds. Great stuff, though it looks pretty crap with my Phoenity theme. I like it better than even the livemarks, because it isn't as finicky about feeds.
Go forth and find feeds! This is a list of all lists syndicated to Livejournal at this moment, which is pretty useful in order to find the popular ones. You could get em on your friendslist, but I still prefer the above methods.