These are some of my favorite extensions. They aren't in any particular order (well, actually, alphabetical because I was going down the Add-on's window as I wrote this), and they're all great to me.

Download Statusbar - One of the neatest space savers for your browser, especially if you download a lot (I do). Instead of staring at that Downloads window, look at a downloads bar, which appears above your status bar. As you can see in the screenshot (-->), it shows progress of each file as well as the current speed over the remaining download time. This sort of information is better at a glance than in its own window.

Faviconize Tab - Ever find yourself multi-tasking, and then realizing your tabs bar has way too many tabs on there? Especially considering, you know what most of them are just by glancing at their favicon. Well, this extension removes the text, and instead, shows the favicon for tabs that you (by default) double click--saving space. What's also handy is the setting that lets you automatically 'Faviconize' tabs based on their URL. A personal usage for this is Gmail, which I always have open, and it's favicon is quite unique