As with any list of Firefox tools, please do not install all of these at once unless you'd like to slow down your browser. And, honestly, how many notifications do you need to show you that you have unread items in Reader?
AideRSS - Brings AideRSS to your Google Reader, allowing you to see ranks directly in your reader, as well as letting you group feeds by rank. Also allows you to add your own thoughts on each feed.
Better GReader - Combines several Greasemonkey scripts in one extension. Includes things such as skipping the iGoogle choice page, the ability to preview the item as it appears on the actual Web page, shows the favicon for each feed, and several other choices.
Feedly - While actually a start page extension, Feedly will monitor your Google Reader activities to deliver what appears to be your most important news so that you can always keep up with what is most important to you. You can read a more in-depth review by Alana Taylor here on Mashable.
Google Reader Notifier - Shows you how many unread items there are, provides an itemized list, lets you customize where the information appears in your browser, different icon sets, and more.
Google Reader Toolkit - A straightforward extension that adds an icon to your status bar that shows you how many unread items you have, and can open Reader for you.
Google Reader Watcher - Based on a Greasemonkey script by the same author, Google Reader Watcher will check your account at an interval you set and will show you how many unread items you have. It can show this broken down by feed and more.
gReader Comments - If a blog has Disqus installed, you can continue the conversation in Google Reader with this extension.
