Firefox 2 and Firefox 3 let the user add plugins to the browser. There are about a million of them that you can find either on the menubar under Tools and Add-ons or at their website at addons.mozilla.org. The add-ons that any web developer must have are Firebug, to debug the code and make adjustments on the fly; Web Developer, to disable, highlight, display, outline, and validate just about anything on any web page; and a recent discovery of mine Firefox Accessibility Extension.
Although Firefox Accessibility Extension has similar features than the Web Developer add-on, it’s still pretty sweet. Not only does it give the user a toolbar, but also an extra heading on the menubar, between Tools and Help, which I personally prefer rather than having a bunch of toolbars taking up space on my browser. The greatest feature of this extension for a web developer is that you can validate your web page for accessibility. Its FAE Rule Set is based on Functional Accessibility Evaluator 1.0.1 developed by the University of Illinois at Urbana-Champaign.
If you have any mistakes these are listed in a window with a description of the error. Obviously there’s a learning curve where you have to know the terms that the report is referring to, nothing a little research won’t solve. These rules are very straightforward, but not exhaustive. This tools gives you a simple and fair reading of your page and this is already a very good starting point.