Firebug is a great tool made greater with the addition of extensions that do everything a web developer needs except serve coffee in the middle of the afternoon. Taken together, this is one impressive set of doodahs.
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Firebug plus PixelPerfect
One of the coolest things you can do with Firebug is combine it with PixelPerfect, an extension that lets you drag elements around on the page using an onion-skin transparency feature to see where it was in addition to where it is now – instantly, without reloading the page. Some web developers may harbor an objection to messy HTML code results from visual placement, but if the front-end results are exactly as you envisioned, it's worth it to most of us.
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The YSlow extension
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Debugging with FirePHP
Hardly anything works right the first time and chasing down errors can be pretty frustrating. With Firebug's FirePHP extension, finding those maddening PHP errors is easier, and it's especially helpful for code containing Ajax applications. There's a lot more functionality to FirePHP than the scope of this article allows, so you'll just have to trust me – it will blow you away.
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The SenSEO Firebug extension offers a checklist of SEO criteria and rates your page using Google's webmaster guidelines. SEO analytic data allows you to tweak and optimize your content and meta-data to earn the love of all the major SERPs and make your clients happy.
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It seems that whatever the development task, there's a Firebug extension tool to address it, and every one is open source and freely available for download. It's hard to imagine a more useful and comprehensive set of tools for any web developer.