Chris Pederick has developed one of the most useful extensions available to Gecko rendering engine browsers Firefox, Flock, and Mozilla.  Named Web Developer Extension, it has a seemingly never-ending array of tools, hack, and tweaks to aid the contemporary Web developer with otherwise awkward debugging and QA tasks. 

Manipulate CSS, forms, images, links, cookies all in real-time!  Highlight, diagram and walk the DOM in WYSIWYG mode!  Validate CSS, HTML, 508, etc.!  Forget one-off bookmarklets that do preform similar functions.  Web Developer Extension's integration allows for the overlay of multiple functions at once with no risk of conflict as with bookmarklets. It's layout is well thought-out, with page information showing in separate tabs and edit screens appearing in interactive sidebars.  If you weren't convinced that Firefox was the defacto Web Developer's browser, download this extension and there will no longer be a question.

Get it now!