Until there is anything formal from Apple allowing alternative browser engines I’d wouldn’t really expect too many updates from Firefox on their custom engine project.
Firefox
A place to discuss the news and latest developments on the open-source browser Firefox
There are a few rumours that Apple might drop the WebKit requirement soon, due to some laws adopted by the EU, however there has been no official response or comment by Apple so far.
If you like privacy and freedom, you could use Android (degoogled, obviously). I understand the EU is trying to force Apple to become better, but it's still a walled garden much more than Android is, or aspires to be.
As far as I know, companies don't have to comply yet with the Digital Markets Act. That's most likely the reason why the WebKit restriction is still in place.
I google it every so often, but I think it’s dead or a private project for now
I'm looking forward to it too. They have mentioned a few times in the past they wanted this but I think as long as it's not officially possible they are not going to post progress reports. Without having any evidence I'm pretty sure it'll happen but I can understand Mozilla not making any promises until it's possible for sure.
Furthermore it will in all likelihood only be allowed in markets that legislate for it which ultimately will probably be everywhere but not to start with.
We just have to be patient.