That's too late I believe - at least both the Palemoon and Thorium browsers already support JPEG-XL. Firefox could be the first major browser to support JPEG-XL om Linux, which would be really nice.
As far as I understand, the flag only works in the "nightly" development releases, not (yet) in the regular releases of Firefox. Support for JPEG-XL in the regular versions of Firefox would be very nice and I think there are patches from e.g. Palemoo...
+1From an authoring perspective JPEG XL makes much more sense and it's important to see the benefits from a broader perspective than just last-mile content delivery to the browser (even if progressive decoding is a major benefit even there).
A somewhat more in-depth comparison and analysis of the data provided by Google can be found here: https://cloudinary.com/blog/contemplating-codec-comparisonsShort version: Real life data appears to support the case for JPEG-XL not only feature-wise,...
