Not really propriety software - as you & bildung said, it's not a problem if you can just buy a license. I meant anyone working on FLOSS software that isn't GPL compatible, which is probably most FLOSS software. MIT, BSD, etc are very popular these days. And image formats are generally intended to be widely-adopted standards used across different applications.
Most "FLOSS" software is GPL. Almost all is GPL compatible.
But even the FSF recommend sometimes using don't-care-about-user-freedom licenses for strategic reasons, for example driving adoption of a new free codec...
Firefox is GPLv3 compatible. It's quite likely that it will support this format - although, if it's the only major browser doing so, it will not be adopted by most web publishers.
I would be very surprised if Firefox added support. They have been very reluctant to include any additional image formats that weren't developed by Mozilla employees. See for example WebP and Jpeg2000.
Their reasons being things like added security risk, lack of demand, lack of support in other browsers, patent risk etc, which all apply just as much to this format.
The Mozilla Public License is GPLv3 compatible. Firefox also contains code that is under different licenses that is not GPLv3 compatible, and thus firefox is not GPLv3 compatible
Apple and Google won't touch GPLv3, so that means this image format is dead on arrival as a web format.
Sure you can use it locally to compress your photographs, cool. But it won't be a web standard with GPLv3.