... There was a recent identical thread over at the Vista subforum:
Read my detailed reply here... or TL;DR:
1. Use Basilisk52[Serpent 52.9.0] with a SSUAO for Discord to pretend you're using FirefoxESR 60 on Win7+, or
2. Use FirefoxESR 52.9.0 and change its default UAO for Discord, with the aid of extensions, to the one of FxESR 60 (on Win7+).
New Moon (27 or 28) won't work at all, because of no support for WebRTC (used by Discord's voice chat feature).