I love my Firefox extensions. They do everything from telling me if my HTML/CSS is valid & 508 compliant to giving me download times for a webpage in milliseconds. I have an array of them for Firefox to make my job of creating streamlined webpages fast and easy.
And that's one of the good things about IE7 is its ability to be given extensions like Firefox. Though the only one that I have bothered installing so far is the IE Developer Toolbar beta 3.
While not quite as well done as the tools in my Firefox arsenal, it's a welcome addition of similar capability in IE, with some nice features that are missing from my Firefox tools.