Here is a list of some cool firefox extensions. You're already using Firefox, aren't you?
Adblock Plus makes blocking Banner-Ads (including Flash and iFrames) easy - extending the blocklist is just a rightclick away.
The Filterset.G Updater keeps your filterlist updated with a comunity maintained list of annoying banners.
The Web Developer Extension is simply a must! Allows you disable/enable styles, view information on forms and styles, gives you an easy way to validate your HTML and CSS sources and much more through an additional toolbar. Wonderful!
If you need the web developer extension you most likely need the Firebug extension. Once you start using it you'll wonder how you ever got by without it.
ForecastFox display the current weather and a forcast of the upcoming days in your status bar.
Forecastfox is available in multiple languages, and works with major cities all over the world.
ColorZilla is another useful extension for webdeveloper. It helps you to get colorcodes from websites.
Live HTTP Headers is great for debugging web applications and webservers. It lets you see all the headers sent by your browser and the headers the server answered with. Customizable filters let you see only things you're interested in.
Sometimes you need to debug all those errors in Microsoft's Internet Explorer aswell,ieHTTPHeaders is the equivalent for IE that helps you then.
The BugMeNot Extension integrates the bugmenot.com's service into the context menu of of password fields. So getting a login on all these “register for free” pages is just a right click away.
The full name of this extension is “HTML VALIDATOR (based on Tidy)” and is another very useful extension for web developers. It helps you to validate websites with out the need to send the content to external validators. A small icon in the status bar always informs you if the page you're looking at has any problems and Tidy can show you the problem in Firefox' source viewer.
Customize Google adds additional links and features to all services offered by google. For example you can enable google suggest for the normal google page, automatically jump to the image alone in the image search or disable Google Analytics cookies.
Screengrab! will save what you can see in the window, the entire page, just a selection, a particular frame… basically it saves webpages as images.