The current Firefox ESR is v128, which was released in July 2024 and will be supported by Mozilla until September 2025. That passes all our tests.
The previous ESR is v115, which would normally be EOL by now, but Mozilla have exceptionally extended security updates for people on Windows 7-8.1. Right now it looks like they’re going until September 2025, but might extend further.
If this were possible, we would. But unfortunately these three features aren’t possible to polyfill.
Yes that’s true of all three features. They are underlying web platform features which our developers will use to improve Discourse. Users won’t use them directly.