Other than being a fantastic browser, Mozilla Firefox allows programmers to create extensions providing functionality not built into the basic browser. Since I've started using Firefox, I've come to rely on a number of these extensions to make my web-developing life easier, including:

• Web Developer Toolbar - This toolbar has become an essential part of my web development. It has a multitude of functions that let you troubleshoot problems; test your web site at different screen resolutions; edit the page's cascading style sheets (and see the changes in real-time); debug forms, and a host of other great functions. If you don't have the Web Developer Toolbar, you're really missing out.
• Linkification - Does it annoy you that URLs in the text of web pages aren't clickable? Tired of copy-and-pasting a URL into the location bar to visit the site? Linkification will make any URL on a page clickable. It'll even do URLs without the "http://" in front.
• IEView Lite - This extension adds an item to your contextual menu (displayed when you right click), opening the page or link you right-clicked on in Internet Explorer. Great for testing...or those pesky sites (I'm talking to you, MSN) that don't work as well in Firefox as they do in IE.
• Performancing - If you're a blogger, you need Performancing. Allows you to post to your blog without leaving the page you're on. Works with just about any blog software or hosted blog service (Wordpress, Movable Type, Blogger, etc.).
• Google Toolbar - Yep, it's available for Firefox now.
• FireFTP - Full-featured FTP client right in Firefox.
• MeasureIt - Click on the MeasureIt button in the status bar, then click and drag to highlight the item on the page you want to measure. It'll tell you the pixel dimensions.
• ColorZilla - Gives you an eyedropper that you can drag over any item on a page to get the RGB colors and the hexadecimal code (think #ffffff) for that color.
• ViewSourceWith - Allows you to set the editor that's used when you choose "View Source." You can even configure several editors and switch between them easily. Handy for opening a source in your HTML editor of choice (like Dreamweaver or HomeSite).

Enjoy!