I use IE 6 or 7, AOL Explorer (basically IE), and, (most of the time!), FireFox
Must-have extentions for FireFox:
FlashGot - allows you to use almost any 3rd-party download manager, or your normal FireFox downloader, your choice.
FasterFox - the name says it all
All-in-One Sidebar - a sidebar control, inspired by Opera's
IE Tab - Kiss IE goodbye (if you want) when using Windoze Update (although you will actually be using IE's engine inside one of Firefox's tabs)
MR Tech Local Install - allows you to install from local .xpi files, disable annoying 3-sec delay, hack version checks (careful!), ...
Update Notifier - notifies you of updates for extensions and themes, can be configured to automatically install them, among other things.
PDF Download - allows you to download or open .PDF files, instead of automatically just opening them.
Also of interest:
NoScript - allows you to control which website are allowed to execute scripts - very handy, when you allow your favorite site, but not its ad sponsor, if their ads are just that annoying.
ShowIP - Show the IP address of the current page in the status bar.
Flashblock - replaces flash objects with a button you can click if you really
do want to see the object.
Foxmarks Bookmark Synchronizer - Synchronizes your bookmarks if you use different machines, using the web.
User Agent Switcher - Adds a menu and a toolbar button to switch the user agent of the browser.