Well, a lot is possible - but not practical

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I don't know much about Firefox OS, but as far as I can tell, the only supported language for apps is Javascript. That would rule out a Navit port, as Navit is written in C.
However, there is a compiler that converts C to Javascript (Emscripten). We might be able to use that. Still, Navit relies on several libraries for input and output, which are probably not available on Firefox OS and would need to be replaced, so it would be a lot of work.
tl;dr:
Yes, it's possible, but probably a lot of work, so not in the near future.