Slack uses Amazon Chime as backend Chime is using h264 firefox still can not encode h264 videos in real time.
Other codecs are better, but you have to do more testing and enable those codecs only on calls where all members are using a device that supports it.
Not that your point is wrong, but a lot of devices are too weak to encode/decode H264 as well. It's very recent for me to have access to second-hand hardware with H264 support and still i'm in western Europe where it's easier to come by.
Though as the other commenter pointed out, if you can afford to use hardware en/decoders then it's always the better option.
I'm kinda curious which devices don't support H264