Have you considered just forcing everyone to access your sites via Internet Explorer 5.5?
Ahh the old Nutscrape Aggrivator. Refused to update with Microsoft standards and spent the next 8 years being a pain in the ass for website compatibility.
As recently as 6 or 7 years ago I maintained some apps that forced 5.5 compatibility mode. Because they were poorly architected in a shitty framework and no one was willing to do or pay for or train for a rewritten version. They were finally migrating to .NET when I left. It was the govt so they are likely wrapping up that migration now.