There's really no reason to be mad at them in this particular instance. Their client is Chromium-based (Electron) so they will optimize their new features for that engine first. There's probably less than 5% users who Discord from browser, let alone Firefox, and I think I'm being generous with that number. Additionally, some things are harder to implement (or even impossible) in native web rather than Electron, that has all the NodeJS integrations.
I love pixel art and am a retro nerd, and I still don't like that change.