Here are a few Firefox extensions that I’ve used that I really, really like and feel lost when browsing without them.  I highly recommend all of these add-ons.  (They work in Windows, Linux, and I assume OS X.)

Tree-Style Tabs
This puts all of your tabs on the side of your browser instead of the top.  This has two main benefits:  it’s much easier to see what your tabs are when you have a lot of them open, and it saves vertical space (good for widescreen laptops and monitors).  Lots of options to configure it to suit your needs.

ColorfulTabs
This goes hand-in-hand with Tree Style Tabs.  ColorfulTabs makes your tabs all a different color based on the site.  Much easier to tell things apart than with just a single color.

Cooliris
I cannot explain how awesome this is.  It allows you to view video and photo searches on a scrollable 3D wall.  Very smooth rendering.  Allows searches from Google Images, YouTube, Flickr, Facebook, etc.

Xmarks
If you have more than one machine that you use regularly, you need this.  It syncs bookmarks and passwords with as many machines as you want.

ScribeFire
This allows you to write blog posts by simply clicking the ScribeFire icon.  It brings up a rich text editor that offers most of the features that you’d have on your blog, only you don’t have to actually take the time to load up your blog.  You can set it up to post to multiple blogs.  It supports Wordpress, Blogger, etc.  Has features for promoting and monetizing your blog posts.

Adblock Plus
It blocks ads, and it does it well.

FlashBlock
This keeps Flash videos/animations from loading and replaces them with a play button.  Hit the play button and the video/animation plays.  Helps websites load faster, and also prevents annoying movies from automatically playing.  Has a whitelist that you can add sites to.

Hide Menubar
Just what it says.  It hides the menus like File, Edit, View, etc.  This gives you a little more vertical space on those widescreen monitors.