Firefox ESR's current version is based on version 78. However, the Firefox (Rapid Release) v.78 has Basic support. Does this mean that Firefox ESR's version has Basic support, not Modern?
Topic on Talk:Compatibility
It'll end up in Grade X, getting Grade A features but untested.
What's the traffic of Wikimedia users using Firefox ESR? I thought about writing a Phab task about this.
Found browser hierarchical view saying that 0.63% (i.e. less than one percent) use version 78. That's much less than IE11 traffic.