Firefox browser have lots of addons. Which can be used to extend its functions as a web browser for a better web experience.

Listed below are some addons that is a must have for every web developer/ programmer:

SQL Inject Me
Tamper Data
Hola Unblocker
FireBug
Live HTTP Header
SQL Inject Me
SQL Inject me is an exploit addon. It is used for testing for SQL injection vulnerabilities. This cannot be used to exploit the site vulnerability. But, display that it exists. With this addon, developers/programmers can view, modify, edit, add or delete records in a database.

Tamper Data
Tamper Data is a great addon for firefox. An excellent tool for security conscious application developers. They can test how their application responds to client side data manipulation attacks. It acts as a bridge between the website and the users. Giving the developer “a behind the curtain view” and play with the HTTP mojo. All those GETs and POSTs can be manipulated without restrictions by the browser interface.

Hola Unblocker
Hola unblocker addon allow access to blocked sites from any country for free. Hola works by masking your IP address to a specific country. This amazing addon have the following features:

Bypass internet censorship.
• Safe and fast browsing experience.
• Reduced bandwidth/data consumption.
• Improved online privacy.

Firebug
Firebug is a free addon for firefox. It facilitates live editing, debugging and monitoring of any website CSS, HTML, DOM and javascript. In addition to debugging, firebug is also a useful tool for testing a web page security and performance analysis.

Live HTTP Header
Live HTTP Header for firefox displays live header of each HTTP request and response. It can also save header information with just a click. And also, allow web developers/programmers to perform a sniffer trace on their HTTP header.
Source: http://www.allspired.com.ng/5-firefox-addons-every-web-developer/