The Mozilla triage meeting in September, which covered this issue, suggests that the requested feature may land in Firefox 58: https://docs.google.com/document/d/1pw5y-GHwDLPV9bYK4HWCiZts...

It looks like it's been broken down into a number of separate issues since then, and there's some discovery yet to be done on how exactly the implementation will need to proceed - I feel like Firefox 58 is very optimistic, but I also feel like saying "this is never going to happen" is pretty premature at this point. At the very least, the Firefox devs don't seem to agree.