Firefox extension for developers (beta)

I want to list the Firefox add-ons that I commonly use related to developing. Some of them are very well known, but some are kind of new ones.

1. Web developer toolbar: This might be the most famous, maybe the better firefox extension for web designers and developers. It can do too many things impossible to list in a single article. There is a nice article of how to check the accessibility of a web site using this tool.
   


2. Firebug: This is the another well-known Firefox extension that compete with the web developer toolbar. You can edit, debug, and monitor CSS, HTML, and JavaScript.


3. Javascript debugger: Powerful JavaScript debugging environment for Mozilla based browsers.


4. YSlow: Analyzes web pages and tells you why they're slow based on Yahoo's rules for high performance web sites. It gives you: Performance report card, HTTP/HTML summary, List of components in the page, andTools including JSLint.
   


5. Live HTTP Headers: View HTTP headers of a page and while browsing.
   


6. Modify headers: Add, modify and filter http request headers. You can modify the user agent string, add headers to spoof a mobile request (e.g. x-up-calling-line-id) and much more.
   

For design related extension, you can check here.

.... still writting