Addon's playing important role for web developer as well as for web desginer also , you can say its make your task easier. Firefox is one of the free and open source web browser and provides most comman features for web devloper and designer,
There are huge no of lists of addons or extensions . here i have mention some of them which are really useful for a developer as well as for designer.
This addon very much useful for the again developer to test there live application in terms of html, css, dhtml , images and so on, ones you install this addon you can find that addon on new toolbar
its enable you to log to your Firebug console using php method call. its basically suit for the ajax development process.
ColorZilla will get you the color which is reading from mouse pointer , you just need to move your mouse on that location and you will get the color of same , you can also zoom that page you also measure the distance between the any two points . you can also save that selected color as a custom palettes
its a very common tool ones you hover on any element, it will display all CSS styles for that element.
Resize your Browser to various size, basically its useful for testing the your application on various sceen , then you come to know you application is feet in current window or not , there are some resoultion : 64x480 , 800×600, 1024×768, 1280×800, 1280×1024, 1600×1200
its a cool addon , you can easily browse the photos and videos from web.
displays the number of found errors of the site you are viewing. it will parse the html page and display the error.
it corrents the typos in URL that enter in the address bar. for example if you type rediff.con instead if com it will automatically fix it and parse it and redirect to you on actaul site, Current version is support the .com,.net,.org.,.edu..gov,.mil and other TLD.
FEBE (Firefox Environment Backup Extension) allow you to quickly and easily backup your firefox. it will rebuild your extensions into .xpi files.
its allows you to analyze your http header , it will also help you to debug you web applicaiton ,you can also see what sort of server is using and also view cookies sent by the remote sites,
stumbleUpon is a social bookmarking sites , its useful to you to search the sites , this site derive a large no of visitors to your sites.
font finder tools helps you to find the font of highlighted text, you need to just select the text and right click to see all the styling get included on that text you can also view the Color, font-family and so on
Validate your page using W3c css validator. just right click on page and select Validate CSS it will validate and parse your page and open result on new tab and show you error on same.
auto fill is one of the best option or addon, instaed of typing text again and again you just save that text on AutoFill Form and whenever you required you simply use that so it will get fill that field automatically , for example on Gmail.com , you need to enter id & password to access your account now instad of typing id & password daily you just save . and ones you open gmail.com page click on AutoFill option . it will automatically get fill your id & password .
if u know more add in on a Comments .