It's not quite Spring yet, but with the release of Firefox 3.6, now would be a great time to give your browser a good cleaning.
First, make sure you're on the latest Firefox. In the Help menu, select Check for updates... and apply any updates offered.
Next, let's look at your extensions. Go to Tools -> Add-ons... and select the "Extensions" tab. Scroll through your list of extensions and if you see one that you're not using regularly, go ahead and click the Disable button. This won't remove the add-on, it'll just stop it loading up when you start Firefox. You can always re-enable it if you decide you need it back. Next, look and see if you have any extensions that you disabled in the past that you probably won't be using again. Let's remove those. Simply click the Uninstall button.
Go ahead and restart Firefox at this point. You might already notice a big speed-up in Firefox's start-up time.
Now that we've taken care of extensions, let's look at your plug-ins. Plug-in vendors are notorious for adding plug-ins to Firefox without asking you so you might have a few there you don't need or want. To check, go to Tools -> Add-ons again. This time, select the "Plug-ins" tab. Scroll through that list. In my experience the only plug-ins that are really useful are Flash and Silverlight. (and the "Mozilla Default Plug-in"). Unless you know you need it, go ahead and disable any other plug-ins in the list. This will probably help Firefox perform better and certainly keep you safer. You can always re-enable the plug-in if you find that you need it later.
Go ahead and restart Firefox so these plug-ins get unloaded from your browser.
One final clean-up I like to do is to "vacuum" my history and bookmarks database. There is an extension called Vacuum Places that makes this really easy. Once you install the extension and restart Firefox, you should see a button in your statusbar that will trigger the clean-up. You should only have to do this once (and you can then disable or uninstall the add-on) because Firefox 3.6 now does its own periodic cleaning. This will just give you a jump so you don't have to wait on Firefox to do it automatically.
With those simple steps, you'll probably have a much faster, more stable, and overall happier Firefox.