> This snippet was an experiment to provide more value to Firefox users through offers provided by a partner. It was not a paid placement or advertisement.

I don't know about legal liability, but you don't really need to look further than this to know that Mozilla lied because these two statements are already inconsistent. "Offers provided by a partner" are advertisements. The quid pro quo in this case wasn't money, but rather a link on booking.com to download Firefox.