It makes no sense to me to use Elixir without running on the BEAM. Absolutely zero. You lose OTP, for one thing, which means that the entire programming paradigm goes out the window.
I don’t think Firefly gets rid of the OTP.
OTP is a programming library/paradigm that doesn't work without the ability to create extremely lightweight threads, which is what the BEAM provides.
BEAM isn't the only runtime possible with green threads. OTP is just the libraries.
Lots of people have written alternatives to BEAM. The only problem they run into is that BEAM is very good, and would be tough to beat. I was an admirer of Erlang on Xen: https://github.com/cloudozer/ling