Ok, going through all the plugins/extensions shipped with a standard Fremantle install:
The solution for adobe-flashplayer is to somehow re-implement that npapi function in the post above. This will also solve all the problems for tablet-browser-mediaplayer-plugin and tablet-browser-default-plugin and allow them to work as-is.
For libssoautologin, we need to figure out just what it is for and whether keeping it working matters. And if it does, we will likely need to clone it so it can be made to work with the newer Gecko version (finding all the Gecko interfaces it uses and ensuring those are still ABI compatible is an exercise in futility given that interfaces are no longer frozen in Gecko)
maemo-geolocation is open source so we can port that.

That just leaves Nokia Maps. We can A.Drop support for it completly (not good given that people like me use it), B.Clone the whole thing and port it to the new Gecko (not really an option given that npatlas.so is over 4mb in size AND makes a lot of calls to the Gecko APIs), C.Find a new maps app that is just as good as Nokia Maps (including the nice offline caching and storage stuff) or D.Find a way to have maps use the old Gecko and everything else using the new Gecko. This should be possible given that maps is the only thing talking to Gecko directly (via browser-eal) instead of going through browserd. So we would replace browserd with a new one that talks to the new Gecko version but we keep the old Gecko version around (including browser-eal) for maps.