Firefox is a popular browser among web designers because of all the helpful add-ons you can install. Here are my top 3 favorite add-ons that I use the most.
This one is ultra simple but extremely useful. All it does is give you a ruler to measure (in pixels) the size of anything on a web page. Proper sizing is crucial to web design, so this tool comes in handy a lot.
Another thing you work with a lot as a web designer is color. The best part of this add-on is the eye dropper tool that lets you find out the exact color of anything on the web.
This is by far the most essential add-on for web designers. You can inspect the HTML and CSS code of any element on a page (and also modify it and preview in real-time). This is really helpful for finding out what styles will affect certain elements, and fixing things when they don’t work properly.
*EDIT: I don’t use this add-on anymore. Instead I use Firefox’s built-in developer tools.