The videos on your homepage are encoded with HEVC which can't be viewed in Firefox. Please consider using an open codec like AV1.
What OP meant is that they would like an open format on the website, which, then, can be viewed in any modern browser. I think that caniuse is a good resource in this regard.
WebM with VP9 video is a good general browser target I think:
But funnily enough, even though h264 is not open, it's a widely decoded video format as well:
I hope open standards keep winning. Overall, everyone wins with the infrastructure being openly accessible, especially the common folk.