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.

File upload is not a chromium feature, it’s a super old basic feature. It’s just their pittiness and upcoming drm implications. I bet if you set your user-agent to chrome it woould work just fine.

This dialog doesn’t do just file upload, after you upload you can cut the sound file into a 4-second clip, inside the client. My bet is that it might technically be possible to do it in Firefox, but not with the same exact code as with chromium, and thus they decided they don’t care.