Firefox nightly has support according to https://jpegxl.io/tutorials/firefox/, of course you're wrong that nobody is attacking FF, but given its tiny niche compared to Chrome it's obviously much less consequential, so the volume attacks on Chrome would dwarf anything FF-related (Safari was also criticized, and they've recently added support)

> after significant time of noone using the format

That's also fasle, this is too new of a format for any significant time of no use to materialize, besides, requiring flags that vast majority of users will not enable is a huge factor limiting widespread use