Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
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).
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
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.
Firebug extension to view which CSS rules are actually used.
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.
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.
Colorize entire web pages manually or automatically. Comes with 28 customizable themes that transform hue, saturation, lightness and opacity. For Firefox 28.0 and above. NEW: Copy themes and group color properties using drag-and-drop.
Check and editing the image maps inside the web page with magnifying glass.
Logging support for web developers (PHP, Python, ColdFusion)