So I'm not waiting for the reimaging to complete after all. I made another Firefox package with SetDefaultBrowser turned on and the desktop icon enabled, set it to push out, and turned off the IE desktop icon via group policy. Every computer will switch to Firefox being the default browser the next time they're rebooted. You're welcome internet.
I have to say using IE Tab Plus made me much more confident in this. We do have at least one major IE-only site and having it Just Work for users automatically and transparently is a major plus. I don't have to send out e-mail explaining how that one has to be visited differently and get the inevitable calls from the people who forget. It Just Works. Less hassle for them, less hassle for me.
Although this is technically a blog, it's primary content is a series of articles on how to get Firefox working in a corporate Windows environment. Later ones build on earlier ones, so you might want to use the Table of Contents on the right to read through it chronologically instead of reading straight down from here.