So, one long-term possibility I'm toying with is using OS X's implementation of X11. I could run Firefox in a modern Linux distro (probably a VM on the same machine, although it could be a different machine), and use X Forwarding to make it appear as a normal app. I think this should theoretically work for a lot of OS's, from the earliest versions of OS X to decade-old copies of Linux.
Has anyone used this approach for old machines?