[shadowgovt]

If the platform is the web, but a standards-compliant site can work in Chrome but not Firefox, the issue isn't standards compliance.

And it's up to the individual site author whether retaining that last chunk of Firefox-only (or Edge-only, or Safari-only) users is worth the cash outlay to test on those platforms. I've done multi-browser development; it adds significant friction to the process, especially if one's testing infrastructure is rocky (and most are, and it costs resources to do better than rocky).