I feel like this is a running joke at this point. I've been waiting on this since getting a mbp in 2003. I swear it has been under development the whole time. Or, at worse, blocked by rendering engine updates. People always seem hopeful the next major rendering update will make it usable.

https://bugzilla.mozilla.org/show_bug.cgi?id=789906

Bug opened seven years ago. It looks like the only real updates in the last year are a non-dev advocating for it.

Good to hear it's getting some attention behind the scenes. I'm a little sad the driving factor is probably just mobile. Given that it already seems to work in the mobile Firefox Preview, apparently using gecko, it gives me a little hope we may one day see this on the desktop.