If you need to develop an extension for a browser, you will definitelly develop for Mozilla products: easy, low-cost, and low-tech.

For IE, you need: Visual Studio 2005, and learn C++… and registry touches, and COM and…

For Firefox, open Notepad, save as .xul, edit an .rdf, know some JS, pack them as .xpi, install it, and:

Hello World!

IE, please, don’t hurt the web.