Its probably a good idea to get in touch with Jeff Shepherd over at the Amizilla project then (http://groups.yahoo.com/group/amizilla/), considering just how much work he's done to bring NSPR to AmigaOS 3.x. I'm not sure how much of his work will be compatible with OS4, but its got to be a good start.
BTW: I think we have a lot of people here... But anyway, I mean, both, OOo and Mozilla (or Firefox) would be really cool. So, what if some extend this project to support Amizilla, as well ? Or at least we define some sort of cooperation ?