As I was told, Firefox Extended Support Release (ESR) will be treated as an Unknown browser (Grade X). In other words, it won't be given basic or modern support, and it won't be treated the same as Firefox. Firefox ESR is different slightly from Firefox (Rapid) because ESR is designed for mainly enterprises, organisations, schools, and others. It also uses each LTS version, the latest being 78 ESR, the final version for macOS 10.9 (Mavericks), 10.10 (Yosemite), and 10.11 (El Captain).
The first version of Firefox ESR was version 10. While Rapid moves from major version to major version, ESR receives security updates and bug fixes without moving to another major version for at least months or one year. The least I can request is basic support (Grade C) for ESR. First, compatibility of versions of Firefox ESR with the core MediaWiki infrastructure shall be tested out. Then whether to give the browser basic support (grade C) shall be decided.
According to "data from 2020-11-28 to 2020-12-09" (adjusted from default date range), 0.67% of Wikimedian users use Firefox 78. That's less than one percent, but also more than those using either Safari 11 and 12 combined (to this date grade A), Edge Legacy 18 (grade C), Edge Chromium 86 (grade A), or Opera 71 (grade A). (I'm still astounded by combined percentages of those using pre-86 versions of Chrome.) I predict that many (or some) Windows 7 users wanting to stick with the same unsupported OS will migrate from Chrome to Firefox when Google ends support for Windows 7 in 2022, resulting in a bearable (if not big) pie slice.