Sorry, not gonna look around for random Youtube videos, I can already tell you that Firefox does not currently support any other interface for video decoding other than VAAPI which the Raspberry Pi does not currently have drivers for.
However, you might be lucky with Chromium. There is a written guide you can find here: Raspberry Pi 4: Hardware accelerated video decoding (GPU) in Chromium - LeMaRiva|tech. This is supposed to work on the current Raspberry Pi OS (though I have not tested it, so don’t take my word for it). Now, I don’t know if the version of Chromium shipped with RPi OS is a special patched version specifically for the Pi and if so, if Manjaro ships the same version, but you could at least give it a try.