They may have tried to open arbitrary mp4s and had Firefox not render it.
Firefox seems to only render MP4 files that use yuv colorspace and aa3 audio channels, which require specific ffmpeg flags during transcoding. It took me a day of grinding whackamole to find the magic set of arguments to make all recent, popular browsers actually display a video:
More specifically, most devices will want _4:2:0_ Y'CbCr. It's highly unlikely that FFmpeg gave you anything that's not Y'CbCr (H.264 can technically encode RGB, but I've never ever seen it in use). And if it chooses 4:2:2 or 4:4:4, you get a warning.
FWIW, if you really want to encode your video in Rec. 2020, you'd almost certainly want a 10-bit encode, and then you're outside what e.g. iOS devices will render again. Rec. 709 is the safe choice.
Some of this is also probably due to using ffmpeg directly, tbh. It's very happy to produce irrational combinations of things that are technically allowed by specs but not implemented anywhere in practice, often exacerbated by it trying to preserve metadata / colorspace / etc where technically possible.
