Presumably though instead of FF dev team making a Moz sponsored extension Telefonica came to them and said "we'll give you this barrel of cash if we can put our name on a new part of FF that gets default installed".

Indeed Moz could have just had Telefonica named as devs on an extension and shipped it as default - which would seem more natural. The way it is makes it more like a marketing move to hijack FF as a place to put an advert.

Appear.in seems to work fine and without messing with my browser chrome.