Firefox is a pretty good web browser. But when you start putting in a few add-on's it becomes a GREAT web browser. Here is a short list of the extensions which I rush to install after putting a fresh PC image together.
1) Ad-Block Plus - Blocking out advertising is essential for surfing the web these days. Ad-Block basically filters out advertising and shows you everything else. You will no longer be subjected to those "pop-over" ad's. MySpace pages load on a dime. You need this.
2) StatusBar - I don't know why Firefox doesn't just include this feature as part of the browser. Rather than having a separate download windows - this add-on will keep your running downloads in the status bar of your browser. That way you can see how much longer you have to wait on something and keep surfing. You will no longer need that damned notification pop-up that locks up the browser momentarily.
3) Stealthier - Apple's Safari web browser has always had a "private surfing" feature which basically lets you surf the web without keeping any history, cookie, or cached files. This is great for using public shared terminals, Internet banking, surfing for bit-torrents at work, etc. If you want that same functionality in Firefox you can use this add-on. You check the box when you want to start surfing without history, and un-check it when you're done. There have been other add-ons with this same concept in the past, but this was the only one of them to keep up when Firefox 3.0 came out.
4) Classic Compact Theme - I use a little Asus eee PC for a lot of my web surfing. The screen is tiny, so real estate is very valuable. To maximize that little screen, I remove the bookmark bar and status bar. I usually also use the "small icons" feature of the navigation bar. But if you are counting every pixel, you need this classic compact theme.
5) GSpace - GSpace turn your Gmail account into file storage. That being said, it makes a huge mess of your mailbox and you should consider opening a new Gmail account to use just for this purpose. My friend and I share an account and often share music, applications, pictures, and more using our mutual account with Gspace. You can also use it as a way to back up files for safe keeping. I don't know how happy the Gmail folks are about this obvious abuse of their service - but I have been using this for a couple of years now.