And Firefox will keep up with the web anyway, almost without additional work from Peter Naulls. So most of his work will probably go into making the RISC OS version look and feel more like a RISC OS application.
As for Flash support, there are two possible ways to get that with Firefox on RISC OS. One way would be to obtain the source from Macromedia, wich IIRC is granted for free to non-commercial developers under NDA. The more desireable way would be to support the GPLFlash2 project: [link] It aims for Flash 7 compatability.