Firefox is my browser of choice at least for development purposes. There are many reasons for that, one is that Firefox offers a Firefox Developer version that's tailored for developers like you and me. Folks at Mozilla have explained it better here https://www.mozilla.org/en-US/firefox/developer/
The Firefox add-ons store is rich. There's range of useful plugins and extensions available for every need. In this post, you'll learn about few of those helpful add-ons for Web Developers & Designers.
You can also check out this list of useful plugins
The Web Developer add-on for Firefox is an all-in-one package of useful tools for web developers and designers. It has many built-in common features for everyday development needs including abilities to manage Cookies, CSS, Images, HTML elements, iframes and even abilities to validating CSS & HTML. It's simple, easy and powerful.
The User-Agent Switcher and Manager is one of the popular add-ons also a recommended tool for Web Developers that enables changing the User Agent String, the browser identifier that is sent to servers. When you update the string, it would appear as a different browser to the server. This tool allows you to set the User Agent randomly or based on website.
Flagfox is another powerful add-on that reveals server information about the currently loaded website like IP address, server locations, DNS information and more. It can track and display a multitude of things right from your browser and also adds a cute little country flag based on the server location of the current website.
Stylus is another powerful addon that lets you customize or redesign the appearance of a website based on user-defined styles. There are lots of user styles available on https://userstyles.org/ for most of the popular websites on the web that you can import through Stylus onto Firefox. For eg., you can use Stylus to customize youtube or facebook or any other site of your choice. This tutorial covers everything about stylus.
The followig video explains how to use stylus and what advantage does it bring to you. It also teaches you how to build custom themes and few a few more things. Take a look.
The FontFace Ninja as the name suggests is an addon to identify all the fonts used on a website and not only does it show the font used but helps you test the font with custom text right in the browser itself. You can also hover over the various text and find out the font faces used. The addon also lets you buy premium fonts or download a font if it's free.
Load Time is another powerful Firefox addon, that displays very detailed information about page load times in the browser's address bar. This is very helpful for tracking and optimization of website load times.
Link Gopher is another powerful plugin, that extracts, sorts, remove duplicates and displays all the link in a new tab from a webpage and lets you copy and keep track of it. It also highlights different domains used.
The HTML Validator is another powerful addon for Firefox that helps validating HTML and display errors right from the browser window. The errors are highlighted when browsing the source of the page. The thing is, it does not send any data to W3 online validator, it does it all by itself.
Whatruns is another powerful add-on that identifies and reveals various technologies used on a website. It can track the scripts, styles, fonts, frameworks, server information and various other components of a website.
HTTP Header Live is another powerful extension, that let's you analyze or edit a website headers and also sends it to the server.