Wappalyzer is a browser extension that identifies software on websites.
Color tools for web development.
Color picker and eyedropper + saving colors and trying out colors with drag and drop.
CodeBurner is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS.
Show the IP address(es) of the current page in the status bar. It also allows querying custom information services by IP (right click) and hostname (left click), like whois, netcraft, etc. Additionally you can copy the IP address to the clipboard.
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)
this add-on simulates japanese mobile device.
this add-on support DoCoMo, Au(KDDI) and SoftBank device.
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.
Firebug extension to view which CSS rules are actually used.
Collector's Note (fr) Gère les @import. Looks like doens't support UTF-8 (i.e. display of Chinese characters)
A really generic user agent switcher, no restart required.
FontFinder is created for designers, developers and typographers. It allows a user to analyze the font information of any element on a page, copy any piece(s) of that information to the clipboard, and perform inline replacements to test new layouts.
Collector's Note "Easy and fast to use - use the eyedropper tool OR highlight and right click. And it gives me exactly the info I need: font stack vs. rendered font, size, color, spacing - without overloading me with info I don't need or want to search through" Open source (GNU)
Dust-Me Selectors is development tool that scans your website to find unused CSS selectors.
Collector's Note Open source (MIT)