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I like these “Best” lists that you find on blogs, so I thought I should at least start contributing my own. And to begin with, my own list of swizzy Firefox add-ons that will help the humble developer.  They’re all Firefox 3 compatible and they’re all free.

In no particular order (other than, erm, alphabetical. Which is an order)…

Firebug
A Yahoo provided plug-in that allows you to edit, debug, and monitor CSS, HTML, and JavaScript live in any web page. Once installed you can couple this with YSlow, a plugin for Firebug, which puts into practise Yahoo’s very own website performance principles – displaying how well the page does and providing a breakdown into what makes up the page download.

IE Tab
With a click of an icon you can switch Firefox’s rendering engine to IE and back again. Useful for those quick cross-browser tests.

LinkChecker
This scans the current web page and highlights each link it finds according to whether it was valid or not – a great way to quickly check link validity on a page.

Live HTTP headers
Allows you to see the header information sent by a web page – particularly handy for checking on compression, caching, cookies and the like.

MeasureIt
This is a quick and simple measurement tool. Normally, I’d recommend standalone apps for this kind of thing so they’re not restricted to the browser, but this does this particular job so well, it’s still a requirement.

RankChecker
When it’s time to keep an eye on your SEO, this is invaluable. You type your chosen search terms into the RankChecker screen and it will display, for a number of popular search engines, where your site came in the results. Great for monitoring whether your SEO is working or not.

Screen grab!
This does a single job but, god, does it well. It takes screen shots from the browser. You can either take an area, the current visible portion or the whole page (whether currently on screen or not). You can then save the results or stick it on your clipboard.

SEO For Firefox
This displays various SEO information about the site – PageRank, links, etc

Split Browser
This is a new one that I’ve come across when I had a specific requirement. Basically, this lets you split the Firefox screen into multiple “windows” into which you can load different pages. In my case I found this invaluable when comparing two pages side-by-side (an existing version of a website and a new, modified version).

Web Developer
What more can be said about this? If you could only have one add-on, this should be it, with links and tools to everything you could imagine. My most-used part of it is the ability to switch off web page components, such as JavaScript and images. Brilliant.

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