pnhtoolbar is a Mozilla extension for web developers that collects a bunch of useful bookmarklets into a single XUL+JS toolbar. Interfaces to over-the-web validators, hide and show style sheets, apply external style sheets, view page cookies, and more. Mozilla already includes some powerful features for web development, pnhtoolbar adds a few extra features to a convenient toolbar.

If you're debugging HTTP headers, LiveHTTPHeaders for Mozilla looks handy. Watch HTTP headers scroll by in a window.