Your only real choice is Yandex.Browser 17.4.1 (if you care about Voice Chat), or any Chromium-based browser with a Blink engine version above version 55 will also work generally speaking as a quite stable alternative.
Note that quite a lot of the Chinese browsers use the same backport code - implying that the companies producing these forks all have access to the same repository - simply with their own account customisation and different levels of telemetry, and different but very similar user interface... it also seems like none of them want to launch properly on XP x64, throwing that OS out of the window almost entirely.
Another choice if you're looking for a non-Chromium alternative is Pidgin + purple-discord by EionRobb which a user known as "ScarletWavez" mentioned as early on as 2018 as being useful for XP machines, but hasn't really been covered. Granted, it's not very intuitive but it's something to definitely consider as it would run a lot more lightly than any web browser would. However, they apparently dislike 'unofficial clients' for whatever reason and seem to sometimes ban people for using them if detected. Use at your own risk.
All Firefox versions are basically treated as 'we don't care unless it's a major issue' by Discord, much like a lot of other 'web apps' in development using heavy JavaScript purely geared towards Chromium. For example - quoting has been broken since 2016 for instance and nobody has bothered to mention it to them as they'd only fix it if they were a longtime Nitro user and threatened to boycott the platform. Mypal used to work, however something bogged it down, much like what happened to Firefox 52 ESR which ALSO left Discord completely unusable on Debian without using a Chromium-based browser for over a year too, due to their team switching to the ESR branch in the official repo (Rust issues, IIRC).
tl;dr Don't expect Discord to work on XP/2003 unless you want to use a Chinese Chrome backport, or anything if you're not prepared to have to use Chrome in some way by force.
tl;dr2 Discord's monopoly on niche Internet communities, and the fact it has killed off much of 'personalisation' and the whole classic forum within the space of four years honestly disgusts me.