

Different distirbutrions subscribe to different “key servers” (is that the right term?) to validate that the packages they’re getting have been signed by the right people, and not by Dick Dastardly and his crew. LibreWolf isn’t your typical Linux package, but probably on the same trustworthy level as some of “extra” packages found in other repos. My guess would be Mint subscribes to the key server where the LibreWolf dev’s key exists, and Ubuntu doesn’t because Ubuntu has a very Ubuntu™ way of doing things (I’m being a snob here).
So I think if you really want to use LibreWolf, you will have to manually subscribe to the keyserver where the LibreWolf’s dev key is, or manually import the key yourself to validate the package.
Anyway, welcome to the wacky races
Thanks for the extra context, though I’m not sure why Mint had that key by default in their keyserver and Ubuntu doesn’t