What happened to the effort by Mozilla to put build flags to all proprietary libraries to make it possible for f-droid to include it?
Are you talking about flags mentioned in F-Droid · Issue #162 · mozilla-mobile/fenix · GitHub? That wasn’t Mozilla.
When that will be ready, is it gonna replace this build?
It’s on hold now (read the comments). Not sure what you mean by “replacing this build.”
If the question is “whether the upstream Firefox for Android will ever become free software”, I personally don’t believe that. Of course there are people at Mozilla who share free software values (they even helped us in the past), but Mozilla as a company is quite hostile to those values.