OK. Enter about:support in the address bar, go to the Media section and paste here the value of Preferred Sample Rate.

Then go to Output devices and paste here Rate of your devices.

Pasting doesn't seem to be allowed in that area. Tried it and nothing happened.

Paste it here, on this forum... I'm just asking what is the value there.

Oh, sorry. I thought you were offering a solution. The value shown under Preferred Sample Rate is 48000. Is there no way to change that?

As for the Output Devices section, it shows "default: 48000, support: 48000 - 48000," which is strange because my audio interface supports 44.1k, 48k, 88.2k, as well as 96k. So why does Firefox think my device only supports 48k?

Browser asks your sound card on startup, which is the preferred sample rate and sets that one to the audio stream. If the preferred sampling rate on your device is 44.1kHz that one will be used.

I don't know, how to configure it on your device.

Negative. Even if I restart my computer and set my audio interface to 44.1k before doing anything else, when I open Firefox there's still no sound and when I go to about:settings it still shows 48000.