Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
FirePath is a Firebug extension that adds a development tool to edit, inspect and generate XPath 1.0 expressions, CSS 3 selectors and JQuery selectors (Sizzle selector engine).
Wappalyzer is a browser extension that identifies software on websites.
RDS seo toolbar (pr, dmoz, alexa, seomoz ... ) – is a useful tool for fast analysis of website and page SEO indicators, designed for webmasters and optimizers. And also with a range of useful functions for ordinary users.
This extension can simplify the rule to just specify the URI and Content type and can enable/disable each rule separately.
What's more, it can customize the web pages by user scripts which is similar to Grease Monkey.
Firebug extension to view which CSS rules are actually used.
Change the colors of web sites. With the move of some sliders, you change the foreground and background colors of a site.
Watch the video for a short introduction.
Check and editing the image maps inside the web page with magnifying glass.
Colorize entire web pages by means of advanced controls for hue, saturation, lightness and opacity. White-list web domains for automatic colorization.
Logging support for web developers (PHP, Python, ColdFusion)