https://chrome.google.com/webstore/detail/bnbbfjbeaefgipfjpdabmpadaacmafkj
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Firebug Lite easily integrated into Chrome. Even if I am not a fan of Chrome, because of the omnipotence of Google, I find this tool very useful as a debug facility.
http://code.google.com/intl/fr/speed/page-speed/
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Google releases Page Speed, a YSlow-like firefox extension. Webmasters and web developers can use Page Speed to evaluate the performance of their web pages and to get suggestions on how to improve them.
https://symfony.com/legacy/wiki/sfFirePHPPlugin
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FirePHP enables you to print to your Firebug Console using a simple PHP function call. What makes FirePHP different? All data is sent via a set of X-FirePHP-Data response headers. This means that the debugging data will not interfere with the content on your page. Thus FirePHP is ideally suited for AJAX development.
YSlow analyzes any web page and generates a grade for each rule and an overall grade. If a page can be improved, YSlow lists the specific changes to be made.