I've been using Firefox for quite a while now. During this time I've found quite a few extensions that I really don't know what I would do without. So, without further ado here's my favorite extensions:

• Cooliris Previews - When you hover over a link a little icon appears, put your mouse on that icon and a fully functional pop-up browser appears showing the link you hovered over.
• ColorZilla - ColorZilla is an advanced eye dropper that allows you to choose colors on a web page. It'll show you the hexadecimal number along with the RGB value of whatever color you click on.
• Web Developer - Web Developer is an awesome extension that allows you to see the CSS of a web page (at least this is what I use it the most for), outline tables and table cells, see image details and more, a must have for anyone who does any web development
• Firebug - Another awesome extension that is a must have for web developers. This is basically a DOM inspector that allows you to monitor or edit any web element live
• Foxmarks Bookmark Syncronizer - This extension allows you to syncronize all your bookmarks across various machines
• Google Toolbar - Kinda self-explanatory
• Download Status Bar - Gets rid of the anoying download window and replaces it with a small bar at the bottom of the browser that shows the status of downloads, very handy.
• IE Tab - And finally, this extension allows you to change rendering engines of a web site to the IE rendering engine, another very handy extension.

There are a couple of others that I use, but for the most part the above extensions get installed on every machine I use regardless. I've also recently discovered Greasemonkey but I mainly just use the LDDMonkey script with it. I've also just recently started using the Adblock Plus extension but it does little bit too good of a job on blocking ads and makes me think things on my website aren't working

So, what extensions can't you live without?