Web Developer Firefox extension

This is an essential Firefox extension for website and blog owners. The plugin will add a toolbar to your Firefox, containing an extensive list of trivial and advanced features. Among other things, the Web Developer extension will allow you to:

disable the cache, cookies, meta tags, redirectors, colors, etc.
display image attributes, link details, access keys, forms, etc
get information about the CSS, divs, anchors, block size, etc
validate CSS, HTML, feed, and links
resize the window screen
outline tables, frames, block elements, etc
You can download it for free here.