Isnt a Firefox plugin just a javascript script?
If so, would that be hard to implement?
Even so, after porting the whole browser, dont you think it's plugin system would be a little easier?
A plugin can be as simple as a javascript file and some XUL interface stuff, or it can be as complex as a set of binaries.
Heck, google are working on something called
Native Client (NaCl) which is an x86 native environment that allows plugins to be written directly for x86 but without caring about the specific platform's OS or even the browser, provided it supports the currently accepted norms for binary plugins.