I'm playing around creating a Firefox add-in (as well as looking into an IE plug-in). These pages describe how to do it (it shows how you can use Visual Studio to help). Lots of ads which is a bit annoying but shows how it is done.
Mueller, J. (2006) Creating Your Own Firefox Add-in with Visual Studio. Retrieved June 7, 2008 from http://www.devsource.com/c/a/Using-VS/Creating-Your-Own-Firefox-Addin-with-Visual-Studio/