One of the best features of Firefox is that it can be expanded with a wide range of optional extensions. There are extensions to help with blogging, privacy, working with RSS files, or just having fun. Here’s my list of the 5 extensions I find most useful when doing web design work:

# “Web Developer”:http://chrispederick.com/work/webdeveloper/
Adds a menu and toolbar with many useful tools. Simply being able to open the CSS of a page for live editing makes it essential any web designer or developer. This extension is like an entire toolbox of extensions, and provides “numerous useful features”:http://chrispederick.com/work/webdeveloper/documentation/features/.
# “GreaseMonkey”:http://greasemonkey.mozdev.org/
Greasemonkey’s user scripts let you customize any aspect of a web page’s design or interaction. Hundreds of user scripts, for a wide variety of popular sites are available and very easy to install.
# “del.icio.us”:http://del.icio.us/help/firefox/extension/
This extension integrates “del.icio.us”:http://del.icio.us with Firefox. Del.icio.us is an excellent way to find new tips, tutorials and resources — which are important for any web designer or developer.
# “Aardvark”:http://www.karmatics.com/aardvark/
When activated, this extension outlines and provides class or id names for any element under the cursor. You can then choose to remove or isolate specific elements on a page. The most useful feature is being able to quickly view the source code of the selected element.
# “Colorzilla”:https://addons.mozilla.org/extensions/moreinfo.php?id=271
Adds an advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies. With ColorZilla you can get a color reading from any point in your browser, quickly adjust this color and paste it into another program.

If you haven’t tried it already — Firefox is free, easy to use, and a relatively small download.