Firefox Add-ons for development and debugging
Switch User Agent
Use it to change the User Agent string of your Firefox browser: https://addons.mozilla.org/en-US/firefox/addon/59
Web Developer Toolbar
A Swiss army knife for every web developer: https://addons.mozilla.org/en-US/firefox/addon/60
View / Remove Cookies
This add-on is very useful when developing features depending on security cookies: https://addons.mozilla.org/en-US/firefox/addon/315, when you frequently need to check cookies of a currently displayed web page (Right click the web page View Page Info Cookies).
Force Content Type
Add-on that allows you to force the browser to use a custom Content Type: https://addons.mozilla.org/en-US/firefox/addon/3207. It is very useful for reviewing REST / SOAP API responses as XML instead of "text/plain" as some systems return.
Script debugger and network sniffer for Firefox: https://addons.mozilla.org/en-US/firefox/addon/1843
Extension of the Firebug add-on: http://developer.yahoo.com/yslow/. Helps you finding performance problems in your web pages quickly and easily.