Lets start a list of your all-time most useful mozilla firefox addons for webmastering. I'll start: SearchStatus: https://addons.mozilla.org/en-US/firefox/addon/321 - Display the Google PageRank, Alexa rank and Compete ranking anywhere in your browser, along with fast keyword density analyser, keyword/nofollow highlighting, backward/related links, Alexa info and more. InFormEnter: https://addons.mozilla.org/en-US/firefox/addon/673 - InFormEnter adds a small, clickable icon next to every input field in a web form, from where you can select the item to be inserted - no typing required. You can configure it to display your frequently used information such as name, email, address... Good for website submissions to directories, etc. What does everyone else recommend?
Here are a couples of addons that I use: Adblock Plus : https://addons.mozilla.org/en-US/firefox/addon/1865 – You can browse your site if you have any ads without worry. FireBug : https://addons.mozilla.org/en-US/firefox/addon/1843 - Edit, debug, and monitor CSS, HTML, and JavaScript live in any web page. AdSense Notifier : https://addons.mozilla.org/en-US/firefox/addon/500 – Get updates on AdSense earnings within Firefox. Great for the every 15-minute checkers. Load Time Analyzer : https://addons.mozilla.org/en-US/firefox/addon/3371 - Displays detailed graphs of loading time in Firefox. The graphs display events like page requests, image loading times, etc. Colorzilla Firefox Extension : http://www.iosart.com/firefox/colorzilla/ – Hover the eyedropper over a color within Firefox and Colorzilla will display what Hex and RGB value it holds. FireFTP : http://fireftp.mozdev.org/ - Use FTP right from inside of Firefox without having to start up another program.
I have the ones I usually use listed here Firefox plugins dynashox list includes a couple I'll have to look into though.
FireBug - Edit, debug, and monitor CSS, HTML, and JavaScript live in any web page. Really Good Tool To Use
I know there is a addon for web developpers. The addon can enable and disable css of any webpage at run time and analytic the webpage's layout. Therefore it is also very helpful for div/css layout beginners. But sorry, I didn't remember its name.
FireBug ^^ dynashox has told the name before Here's another one: Rank Checker. You can check your rankings for a lot of keywords/keyphrases in a few seconds. A+++
I like SEObook's Rank Checker and Seo for firefox. Measureit and Colorzilla are nice ones for designers as well.
Oh man, where have I been? Just installed 3 plug-ins listed above. LinkDiagnosis is interesting too (extension, not addon)