Well, as I see it, there are 2 basic types of ports...

1.Easy ports
2.Hard / (almost) Impossible ports

For those in first category, why NOT port? In best cases there's no modifications to sources required...

But for second part, programs, like Mozilla... It might well be easier to start new project, taking that open source program as "goal" and start from scratch...

Of course, it wouldn't become 1:1 copy, but in some aspects it might be even better; For web browser, it might be much smaller, and have non-custom GUI (But standard MUI gui instead)

Having said that about ports, I'm working on 2 different game projects for MorphOS.