do have an eGPU, please provide an inxi --admin --verbosity=7 --filter --no-host --width as that would be the minimum required information for us to be able to help you. (Personally Identifiable Information like serial numbers and MAC addresses will be filtered out by the above command)
Also, please copy-paste that output in-between 3 backticks ``` at the beginning and end of the code/text.
do not have one: well: no hardware, no hardware acceleration.
But since Firefox only supports VAAPI for hardware acceleration (IIRC anyway) and there’s no VAAPI driver for the Pi graphics chip that I’m aware of there’s currently no way to use those hardware capabilities unfortunately.
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.