I've prepared the list of useful extensions for FireFox browser. These extensions are very useful for web developer.
My favorite extension is "Web Developer" and "Live HTTP Headers" on bottom of below list.
Add N Edit Cookies
http://addneditcookies.mozdev.org/installation.html
Live HTTP Headers
http://livehttpheaders.mozdev.org/installation.html
QuickJava
https://addons.mozilla.org/firefox/1237/
TamperData
http://tamperdata.mozdev.org/installation.html
FireBug
https://addons.mozilla.org/firefox/1843/
Download Statusbar
https://addons.mozilla.org/firefox/26/
Web Developer
https://addons.mozilla.org/firefox/60/
