FireScope is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS. The extension's core functionality is centered around a new Reference panel, which contains a search tool for looking up HTML elements, attributes, and CSS properties.
Slow analyzes web pages and tells you why they're slow based on the rules for high performance web sites. YSlow is a Firefox add-on integrated with the popular Firebug web development tool. YSlow gives you:
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Install Firebug first!
* Performance report card
* HTTP/HTML summary
* List of components in the page
* Tools including JSLint