Here's a couple of extensions for Firefox that have me excited:
Bloglines Toolkit. This one has been out a while, and I've been using it constantly. It greatly simplifies subscribing to feeds, by adding a context menu that lets you choose to subscribe to the current page (or link if you right-clicked on a link). For the current page, it uses feed autodiscovery to list the feeds available for that site. Even better is that it also provides a notifier. It places a small icon in the bottom right corner of the browser, and adds a red dot to indicate when new posts are added to any of your subscribed feeds. Simply click on the icon to open up your Bloglines feeds list (you can configure it to open in new tabs, active or inactive, same page, etc.) This has made the process of surfing the web and keeping up to date on my favorite sites so much easier. It wouldn't be hyperbole to say that it's revolutionized my browsing experience.
WeatherFox. This was just created yesterday, and has been updated several times. Look for more improvements as time goes. Basically, it adds icons in the status bar indicating the weather for a zip code you supply. You can hover over an icon for more details, and you can also set how many days of icons to show. A handy way to always see up-to-date weather info without having to open up a website.