It sounds like you missed the distinction between a) displays an error and falls back to slower code and b) does not display an error and silently falls back to slower code. Neither of these options break any compatibility with Javascript proper (and yet one of them is highly more desirable than the other).

asm.js does not break backwards compatibility, asm.js code runs exactly the same in all browsers (if it doesn't, that's a bug and must be fixed of course).

Firefox will warn about asm.js validation errors, but those are just warnings in the web console. They do not affect semantics of execution at all, and are undetectable by the running code. So the code will run identically in all browsers regardless of whether it happens to fall in the asm.js subset or not.