While every web designer or developer should realise the need to maintain a working installation of all the major – and some not-so-major – web browsers, such as Mozilla Firefox, IE6, IE7, Opera and Apple’s Safari, it’s undoubtedly the Firefox browser which has become the darling of the web.
It’s the browser that kick-started a whole new and much needed era of ‘Browser Wars‘ and arguably still one of the best all round browsing experiences out there, but it’s also the large amount of often outstanding plug-ins available for the browser which have, in my mind, got much to do with it’s continued success.
Here’s a top selection of plugins for Firefox to aid and assist the lowly web professional. If you know of any others, shout it out in the comments :
Firebug

I recently asked my Twitter followers to shout-out any Firefox plugins which they deemed essential and many were quick to respond : Firebug. Since installing it I think I’ve used it more than any other web design/development plugin I own.
Firebug sits in the lower section of your browser window and allows you to easily inspect, view and edit many aspects of a page ‘on the fly’, to name only it’s primary functions. It’s an outstanding plugin and I’m really not too sure how I survived so long without it.
Download Firebug.
Web Developers Toolbar

While some features overlap with (and are outdone by) the nicer, shinier Firebug, I still think it’s a good idea to have Web Developers Toolbar installed.
Among many other things the Web Developers Toolbar offers quick access to information on images, forms, mark-up and stylesheets, cookies and the W3C validators either via the titular toolbar or (my preferred method) a handy context-menu.
Download Web Developers Toolbar.
Colorzilla

Colorzilla is a beautifully simple yet utterly indispensable little plugin. All it does is place a little eye-dropper icon at the bottom of your browser window which, once clicked, allows you to point and pick colours from within the current webpage. It’s brilliant.
Download Colorzilla.
MeasureIt

Another simple one that I use a lot. MeasureIt also manifests itself as a small icon in the status bar which this time allows you to click-and-drag to measure rectangular spaces on the page. Very useful.
Download MeasureIt
Honourable Mentions
A couple of other excellent plugins and extensions which aren’t strictly for the web-pro, but come in handy all the same :
This one plugs your Firefox bookmarks directly into your del.icio.us social bookmarking account, which means a much more useful bookmarking experience and the accessing the same bookmarks across multiple machines, either by connecting with the same account via the plug-in or on your webpage at del.icio.us. Indispensable.
PicLens, by CoolIris, takes web based photo galleries to the next level by stitching them altogether in a beautiful full-screen swooshing 3D display. It supports many existing picture and photo websites right out of the box, including Flickr, Pixie and even a normal Google Image search, and as David Behan points out on his blog can also be very useful to the web designer in search of the perfect stock image.