Firefox did not take very long to support AVIF (another recent image format clearly favoured by Google). Please do correct me if I’m wrong, but I’m not aware of Mozilla expressing any reluctance over AVIF’s abysmal lossless performance or other shortcomings (some of which have been addressed since), or insisting on using a memory-safe decoder like they have for JPEG XL. Why the stark difference in treatment?

We may not be able to say for certain, but Google’s massive influence is by far the most plausible explanation I can think of. And if the first step in your decision making is looking at what Google is doing, then ‘following marching orders’ is suddenly not such a bad description. (One can argue that with 3 % market share they don’t have much choice, but that’s beside the point.)

[2] was anticipatory obedience from Mozilla. Good for them that they grew a spine later (or more likely: they knew that Google will implement it some time later, so the change of opinion cost them nothing).