IE Tab (http://ietab.mozdev.org/)
I'll admit it; I have been an IE guy for years. From a design standpoint you absolutely cannot ignore the IE browser share. Depending on the type of site you are running, IE can be upwards of 85% of the traffic. This extension allows you to render IE content right in Firefox. While this is pretty cool... its nothing compared to the ability to flip back and forth with a flip of a switch, comparing design details and tweaking on the fly.

Web Developer (http://users.skynet.be/mgueury/mozilla/)
This is Mozilla's answer to the IE Developer Toolbar (or perhaps the other way around). This basically has everything you'd ever want in a developer toolbar. Disable web content, outline areas, dom inspector, source options, sizing, you name it.

HTML Validator (http://users.skynet.be/mgueury/mozilla/)
I'm not a huge stickler for W3C conformance (as can be seen from this site). But if I'm getting paid to design, I might as well conform when possible. This extension highlights your problems and makes suggestions. Note: if you install the IE Tab mentioned above, make sure you are in Mozilla mode before validating.

FireBug (http://www.joehewitt.com/software/firebug/)
Haven't played with this one much yet, but looks like a pretty darn good Javascript, DOM inspector/debugger.

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