I'd say so, for certain values of "modern." Certainly it may be easier to get a good basis more easily this way. After all, Classilla is just cramming new code into an old, known working code base, and VMS Mosaic did the same thing with X-Mosaic (check out VMS Mosaic: George has done some amazing things with it).
A while ago I was inquiring from people who maintained old System 7-compatible browsers to see if they still had the source code. All of them thought that was a wonderful question, and none of them could find any. It's finding a good basis that's hard, and starting from scratch is a lot of work.
I have a few ideas about this, but they're just ideas, really. In the meantime, you could certainly compile Firefox on a 68K running NetBSD. Running it, and running it well, might be another story ;-) That said, Dillo might be a good choice if you could port it.