As a web developer i prefer following add-ons which are more productive for web development

1. FireBug
FireBug Allows you to manipulate various Web Development Entities Like CSS , JavaScript , HTML , etc
2. FireFTP
FireFTP
  1. Working in live server directly . u might be using an IDE , Browser and a FTP Client that allows u to upload content.
  2. what u do ? change Code in IDE , Upload to server , Refresh page in Browser
  3. Imagine U can have FTP in browser itself then switching and processing of one seperate process is ultimately reduced.
  4. Yes FireFtp is an ftp client embeded in mozilla
3. Web Developer
Web Developer
  1. Manage Enable/Disable CSS , JavaScript,Cookies,Forms,Images , HTML , etc
  2. Allows You To Validate CSS , Feeds , HTML , Links etc
  3. It has a Lot More To Look At
4. Image Spider
Image Spider
  1. Highlights all the Image elements in WebBrowser
  2. If U change to current image in addon preferences , The Image Highlighted by mouse movement will be displayed with consecutive information
5. View source chart
View source chart
  1. Allows You To See Source in a Proper XML Format , rather than how it look’s in View Source
  2. Allows You To Navigate Various Tags With a Tree style navigation
  3. i hope u got what im trying to say by looking at the icon.
6. User Agent switcher
User Agent switcher
  1. Allows U to run same page in different modes like iphone , internet explorer 6 7 8 , etc
  2. allow u to look in search robots mode like slurp , googlebot , msnbot etc
  3. the add on developer says there are more user agents on the way
7. Auto Fill Forms
Auto Fill Forms
  1. Some Times U need to reenter similar data into various  forms   , etc
  2. Allows developer to do unit testing in much efficient way etc
8. HTML Validator
Html Validator
  1. Validates Your HTML and shows error information in staus bar2. Use it and Develop Errorless and bug free html pages
9. Regular Expressions Tester
Regular Expressions Tester
  1. Validate , Save Reqular Expressions easily even ehile typing
  2. allows u to validate regular expressions over JVM rather than JS DOM also
  3. it has ability to import,export,save regex’s etc
10. CSS Reloader
CSS Reloader
  1. Doing some CSS Modifications in ur site , dont refresh or reload the site each time to test , just use this one and reload only the css file needful , it will automatically apply new css.
  2. This helps faster designing.
  3. This helps a lot for the developers who has limited download and bandwidth internet connections , save lots of memory (i mean page wait can be reduced while u develop)