At the risk of wading back in when I'm not the one involved in doing the work, I would've thought that the initial port is the hard part. Once that's done then it's (relatively) easy to port newer versions as they appear, provided they don't involve large changes to the original code. As a simple example from someone with very little experience in doing this, I tried porting the Elite: TNK code (before David Braben went on the warpath against it). That used libraries not available on RISC OS for graphics, keyboard etc., so I replaced them with rather crude things I'd put together using the same API. Hey presto, the whole thing then compiled and ran (after a fashion!). Any newer version would've also compiled and ran. Of course, Firefox is a much more complicated beast, but I would imagine that the principles are the same, so when it does appear I wouldn't worry too much about it falling behind.