The Web Developer extension adds various web developer tools to the browser.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
Hinweis des Sammlers Great time-saver - avoids tedious opening and copying and pasting of six-character codes.
YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages.
http://yslow.org is the official open source website.
Hinweis des Sammlers Great for easy to follow guidance on how to optimise pages for speed of loading.
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)
Hinweis des Sammlers Although HTML validation is included in the Web Developer Toolbar, this provides continual, instant notification of validity and helps enormously by quickly directing the developer to the lines of markup in question and providing corrective information.
CodeBurner is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS.
Hinweis des Sammlers Avoids the need to search websites for information on (X)HTML tags, and provides useful information on browser comptability.
Finds HTML elements matching chosen CSS selector(s) or XPath expression
Pixel Perfect is a Firefox/Firebug extension that allows web developers and designers to easily overlay a web composition over top of the developed HTML.
Screengrab! saves webpages as images... but not for new versions of Firefox.
FF changes too fast and I don't have the time to maintain it anymore. Bugs have developed that I am not in a position to investigate.
Therefore, Screengrab is EOL.
Hinweis des Sammlers Takes care of quick and easy copying/saving of pages as images, and provides choices of complete page/frame, visible portion, selection and window.
SenSEO is an extension checking the most important on-page-SEO-criteria and calculating a grade of how good your site fulfills these criteria.
Hinweis des Sammlers Provides quick-to-access guidelines on ensuring your page is optimised for search engines. The fact that all the information is in one convenient place saves a lot of time and headache.