An easier solution would be to enable e10s. It should be on by default in the next ESR, and I know TBB has been working to make their patches compatible with it.
Not just e10s, they also need to enable the sandboxing, i.e. it requires Firefox 50 at least.
It should actually be easier for Tor to enable stricter sandboxing than in the default Firefox, though, as presumably they have to care less about compatibility.
