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"...is not supported in this browser", fuck you Discord
(programming.dev)
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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.
You're probably right. A modern browser that supports webassembly can do literally anything, implementing the missing AudioData functionality should be possible with enough development effort, but it's not important enough for them to make this particular feature works on Firefox.