IE Tab / IE View
These are 2 different extensions that do similar things. I use it mainly to check my work in IE and Firefox without ever launching IE (YES!!!). Internet Explorer runs within the Firefox window. I also use it to view sites which don't display properly in Firefox, because of pathetic developer negligence. Open links in a new IE tab or switch to IE in the current window at the click of a button. I use this everyday.

Web Developer Extension
This extension gets pretty in depth and is a must have… I use it mostly for the validators, the resize window, changing form posts to gets, and especially the CSS tools, which I use as a quick (live) way to trouble shoot complicated sites.

View Rendered Source Chart
This is a great way to look at page source and identify where the parent and child elements are.

Pearl Crescent Page Saver
This fantastic extension lets you capture screenshots of an entire webpage or visible area as a png or jpg. My only complaint is that it cannot capture Flash areas… yet.

ColorZilla
This is an eyedropper tool for the browser. Capture/check colors on any webpage… Again… this doesn’t work for Flash areas.

MeasureIt
Yes, I do actually use this… not a whole lot but enough to justify its install. Quite simply, it is just a dragable ruler within the browser window.

Bookmarks Synchronizer SE
This is how I keep my bookmarks synched between my laptop, my home computer and the box at my office. Automatically synchs a XML bookmarks document to your FTP or SFTP server. It even maintains the favicons and has undo/redo. Very good.