First off, download the Firefox web browser (if you haven't already).
Next, I'm pretty sure you get annoyed by seeing those useless banner ads, annoying GIF's on some web sites. That's where Adblock comes into play. Use this handy extension to block ads, annoying GIF's, flash objects etc. You can even whitelist pages (like mine :D) if you want to see ads on a particular website.
If you are a download freak, check out FlashGot, which integrates with popular download managers for Windows. Another extension, DownThemAll!, is a download manager/accelerator within Firefox. It increases download speed by 400%. How cool is that?
Moving on, Fasterfox offers performance and network tweaks. Using it in the default mode works fine. But you might want to tweak a little bit and see the improvements.
If you listen to a lot of music while browsing (like me), you'd find FoxyTunes very useful. You can control a variety of media players without having to leave the browser with this handy extension.
There are so many other dandy extensions like GMail Notifier, Email Notifier (Yahoo!, GMail, Hotmail, your personal POP 3 account), Restart Firefox (Adds "Restart Firefox" menuitem to "File", and a toolbar button for easy restarting. Great for when you install themes or extensions and need to restart Firefox), WataCrackaz (neat for sending SMS Text Messages around the world). Check out more at the Firefox Extensions page.
With so many available extensions, and many more added everyday, which extends the functionality and use of Firefox, an inbuilt RSS reader, tabbed browsing and more, how can anyone call it a mediocre browser? I'm not going to conclude that Firefox is the best browser out there. I know it is. I leave it to you to conclude if it's good or not. Feel free to comment me and correct me where ever I've gone wrong :)