While I agree with the fact that most JS is not performance critical (which was kinda my point) I'm interested in where these interpreters are, as Mozilla has

While I agree with the fact that most JS is not performance critical (which was kinda my point) I'm interested in where these interpreters are, as Mozilla has had a JIT for ages (for example) ... if they kept their interpreters about surely it would be just for non-jit compilation targets?