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
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.