Do you have some experimental flag enabled or something? Neither link is working for me on LibreWolf.

Regardless, it is good to know that when Firefox supports JPEG XL, it seems to support animated ones.

EDIT: I just enabled that, and both of the above links work as expected!

1. Navigate to about:config

2. Search jxl

3. Toggle image.jxl.enabled to true

Actually, see what @mschae@discuss.mschae23.de posted below. That is likely a safer method than messing with about:config