This is something that I found recently, which I haven't seen mentioned anywhere, so I thought I'd pass it on.
It installs a toolbar in Firefox which allows you to disable a huge variety of things including disable JavaScript and stylesheets, resize the screen to user-defined screen sizes, make frames resizable, disable cookies, validate pages, put outlines round images, see image file sizes, view generated page source (i.e. after all javascript such as document.write has executed) and loads more. I've found it indispensable over the last month or two.
http://chrispederick.com/work/web-developer/
It installs a toolbar in Firefox which allows you to disable a huge variety of things including disable JavaScript and stylesheets, resize the screen to user-defined screen sizes, make frames resizable, disable cookies, validate pages, put outlines round images, see image file sizes, view generated page source (i.e. after all javascript such as document.write has executed) and loads more. I've found it indispensable over the last month or two.
http://chrispederick.com/work/web-developer/