Regardless of the closeness of the plugin specifications, it'll probably have to be redone, since the details of it are specificed for RISC OS. But that's not a big deal, since it's not that complex. And what's this "FireFox" people keep talking about? Must be related to that myterious "Netsurf" that's sometimes mentioned. CamelCase and all that.
Whilst I'm sure that "native" RISC OS support of TTF would be nice, the bottom line is that the RISC OS developers may well have better things to do. Besides, this is the very type of thing that would be relatively easy to add to RISC OS by a third party, and could make use of rUFL which was developed for Unicode font display in NetSurf.