I use Firefox as my default browser and have been working on developing my skills in web development. Here are a few of the tools for Firefox that I use and find very helpful in helping you learn and grow. (NOTE:  I am unable to link you to the exact location on the Mozilla website, I apologize for the inconvenience. There is a link on a few of the extensions to the developer site.) Enjoy!

ColorZilla 2.0.2

This addon features an advanced eyedropper, color picker, page zoomer and other useful tools. With it, you can obtain color information from any point in your browser, quickly adjust the color and paste it into another program. You can also zoom the page you’re on and measure distances between any two points on the page. It also includes a built-in palette browser containing pre-defined color sets and you can create a custom palette.  There are many other features that you can check out on their web site (the second link).
http://www.colorzilla.com/firefox/

Dummy Lipsum 2.3.2

This addon generates ‘Lorem Ipsum’ dummy text. This is useful when creating templates or web sites.
http://sogame.awardspace.com/dummylipsum/

Firebug 1.4.5

This addon provides development tools you can use while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live on any web page.
http://getfirebug.com/

FireFTP 1.0.6

This addon is a free, secure, cross-platform FTP client for Firefox that provides easy access to FTP servers. You can compare directories, sync directories while navigating, use SFTP, SSL encryption, and much more.
http://fireftp.mozdev.org/

FirePHP 0.3.1

This addon allows you to log your Firebug Console using a simple PHP method call.
http://www.firephp.org/

FireRainbow 1.0

This is an experimental addon that requires Firebug. It brings JavaScript syntax highlighting to Firebug.
http://firerainbow.binaryage.com/

MeasureIt 0.3.9

This addon gives you a tool to draw out a ruler to get the pixel width and height of any elements on a webpage. It has many more features, see the MeasureIt web site (second link) to see what else it does).
http://www.kevinfreitas.net/extensions/measureit/

YSlow 2.0.2

This addon analyzes web pages and why they’re slow using Yahoo!’s rules for high performance sites. It analyzes pages and gives suggestions on ways to improve their performance. Firebug is required to use this addon.
http://developer.yahoo.com/yslow/

If you have any suggestions for extensions you use or experiences you would like to share, please feel free to leave a comment and share the wealth with our fellow readers.