A collection of Addons that are great for web developers.
The Web Developer extension adds various web developer tools to the browser.
View HTTP headers of a page and while browsing.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
Collector's Note Great little addon that tells you the colour of any element on the page with its Color Picker. Also lets you select a colour from its Color Palette. Very Useful for quickly grabbing a colour from the page.
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
Collector's Note Adds the ability to measure pages and items on the page by drawing a box. It then displaying the dimensions of the box in pixels. Very simple but very useful.
HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon
For Firefox 23+, you need 0.958 (under review)
Collector's Note A tool to tell you if your page has valid X/HTML code via a symbol in the status bar. Also lists errors in the View Source.
Display the Google PageRank, Alexa rank, Compete ranking and SEOmoz Linkscape mozRank anywhere in your browser, along with fast keyword density analyser, keyword/nofollow highlighting, backward/related links, canonical links, WHOIS, robots and more.
Collector's Note This addon adds functionality such as: - highlighting "nofollow" links - Icon symbolising the Canonical URL - Shortcuts to the Archive.org, robots.txt, sitemap.xml, etc of the current page As well as other great features, some of which are more geared for SEOs such as PageRank.