Top 5 Firefox Add-Ons for Developers

I'm a Firefox user, have been for years. I love Microsoft products and most of their software is fantastic, but I have never been much fond of Internet Explorer. Maybe its IE's UI, or maybe its the fact that it doesn't have many great plug-ins. Either way, I much prefer to use Firefox as my browser of choice when developing web applications. There are tons of great add-ons available to Firefox users to make their web developing lives easier. Here are my favorite 5:

1. Firebug - This one's got it all: JavaScript debugger, DOM inspector, traffic analyzer, CSS inspector, and  much more. If you aren't already using Firebug, you should get it now. It's a great asset to any web developer's toolset.
2. IE Tab - If you're tired of having to open up another browser to test the compatibility of your web page in IE, or having to switch browsers for site which require IE, rest easy with this plug-in. It allows you to open up web site in a Firefox tab using the IE embedded engine with a single click.
3. YSlow - This is a new one for me. It's a cool plug-in which analyzes web pages and then gives you suggestions on how to improve the performance of your page based on Yahoo!'s rules for high performance web sites.One feature that I enjoy is that YSlow integrates with Firebug. 
4. View Source With - Immediately access any source information - page source, embedded CSS and JavaScript, images, etc - from the web site you are browsing. This tool allows you to open these items in your favorite editor with the click of a mouse, and is handy for gaining quick access to the code of a web page.
5. FoxMarks - Okay, so this last one is not really a development plug-in per se, but since most developers use more than one machine, this one is a must-have. It easily allows you to keep all your bookmarks synced between computers, so you never have to go searching for your favorite links and the days of manually porting your bookmarks from one computer to another are long gone.