There are hundreds, if not thousands of Add-ons out there for Firefox. This list attemps to cut through all the BS and spotlights 20 of the best Add-ons for webmasters, to make your life a bit easier. I have personally used all of the Add-ons listed below so I may have included small notes in the descriptions.

Adsense Notifier
Displays your Adsense earnings on the statusbar. Automatically updates periodically throughout the day.
https://addons.mozilla.org/firefox/500/
Firefox 1.5 - 2.0.0.* ALL

Advanced Dork
Gives quick access to Google’s Advanced Operators directly from the context menu.
https://addons.mozilla.org/firefox/2144/
Firefox 1.5 - 3.0a2 ALL

Clear Cache Button
This is a great add-on for Flash Developers. Since Flash websites can sometimes be a bitch and not display changes by simply refreshing the page. It adds a clear cache toolbar button. After installing the extension, find the clear cache button in the toolbar customization panel.
https://addons.mozilla.org/firefox/1801/
Firefox 1.5 - 2.0.0.* ALL

ColorZilla
This add-on saves you a ton of time if your a designer. Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies.
https://addons.mozilla.org/firefox/271/
Firefox 1.0 - 3.0a2 ALL

DT Whois
Allows you to perform a Whois on the current site you are browsing with the simple click of a button.
https://addons.mozilla.org/firefox/2855/
Firefox 1.5 - 3.0a1 ALL

FireBug
Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
https://addons.mozilla.org/firefox/1843/
Firefox 1.5 - 2.0.0.* ALL

FireFTP
Excelent FTP program integrated into Firefox. FireFTP is a free, secure, cross-platform FTP client for Mozilla Firefox which provides easy and intuitive access to FTP servers.
https://addons.mozilla.org/firefox/684/
Firefox 1.5 - 2.0.0.* ALL

Firefox Showcase
I personally really love this add-on, it allows you to manage all the tabs you have open a lot more easily. It shows them all as thumbnails in a single window.
https://addons.mozilla.org/firefox/1810/
Firefox 1.5 - 3.0a1 ALL

Greasemonkey
Allows you to customize the way a webpage displays using small bits of JavaScript.
https://addons.mozilla.org/firefox/748/
Firefox 1.5 - 2.0.0.* ALL

Html Validator
HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon in the status bar when browsing.
https://addons.mozilla.org/firefox/249/
Firefox 1.5 - 2.0.0.* ALL

IE Tab
Allows you to see how your page would look in IE, by embedding Internet Explorer into the tabs of Firefox.
https://addons.mozilla.org/firefox/1419/
Firefox 1.5 - 2.0.0.* ALL

iMacros
I recently discovered this add-on and have been pretty happy with it. iMacros allows you to Automate your web browser. Record and replay repetitious work.
https://addons.mozilla.org/firefox/3863/
Firefox 1.5 - 2.0.0.* ALL

JSView
Allows you to locate and view the source code of external files like javascript files and css style sheets.
https://addons.mozilla.org/firefox/2076/
Firefox 1.5 - 3.0a2 ALL

LinkCounter
Displays a count of total links of a web page and frames in status bar.
https://addons.mozilla.org/firefox/3958/
Firefox 1.5 - 3.0a2 ALL

MetaTags
This sidebar displays page meta-information (metatags, links, etc.) Has automated search engine submission menu. THIS IS NOT FIREFOX 2.0 COMPATIBLE YET.
https://addons.mozilla.org/firefox/1668/
Firefox 1.5 - 1.5.0.* ALL

Palette Grabber
Great for Designers - Creates a color palette for Photoshop, Paint Shop Pro, GIMP, or Flash based on the current page.
https://addons.mozilla.org/firefox/2290/
Firefox 1.5 - 2.0.0.* ALL

SeoQuake
Helps with search engine optimization and internet promotion of web sites.
https://addons.mozilla.org/firefox/3036/
Firefox 1.5 - 3.0a1 ALL

ServerReplace
Allows you to open the same page on different servers.
https://addons.mozilla.org/firefox/1420/
Firefox 1.5 - 2.0.0.* ALL

Total Validator
Perform multiple validations and take screen shots in one go rather than using separate tools. This 5-in-1 validator works with external, internal, or local web pages using the Total Validator service or local copy of the Pro desktop tool.
https://addons.mozilla.org/firefox/2318/
Firefox 1.5 - 2.0.0.* ALL

Web Developer
This is probably my favorite and most useful Firefox Add-on. It adds a ton of different web development tools to your browser. It gives you the ability to edit CSS stylesheets directly in Firefox which is VERY useful.
https://addons.mozilla.org/firefox/60/
Firefox 1.5 - 2.0.0.* ALL

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