Mozilla Firefox has some cool developer tools (Style Editor, Scratchpad, Debugger, Responsive Design View, Inspect(3D view) and more) by default, but you can further extend them with some of the great add-ons/extensions listed here.
Firebug has lot of useful features (edit, debug, monitor your code in live mode) that makes web development much easier. It’s a must have add-on for web developers.
2. Rainbow Color Tools
it provides a bunch of color tools for web development – easy to use color picker, color library, inspector, analyzer(to analyze the color scheme of a website)etc.
It tells you about the softwares(libraries, frameworks, server etc) used by a website.
It shows the IP address of the website, in status bar; additionally, you can also query whois, hostname, netcraft etc.
It’s good for exploring the fonts used on a website (with the font panel or show fonts section in right click menu)
6. Web Developer
add some cool web development tools.
7. User Agent Switcher
Using this extension, you can easily switch the user agent of the browser.
9. Modify Headers
To modify the http request headers sent to servers.
It’s a cross-platform FTP/SFTP client for Mozilla Firefox.