TL;DR: Firefox usually uses AV1. Hardware prior to intel 11th gen tiger lake, amd rdna2 (pre rtx 6000 series) and nvidia 3000 series has no hardware AV1 decoding support (and neither does Apple Silicon). This results in higher CPU usage on Firefox, whereas Safari and Chrome usually resort to codecs that have hardware decode support on the machine.
This can easily be fixed by going to `about:config` and setting `media.av1.enabled` to false.