Mozilla has added search engines to their default list like this before for similar reasons of sponsorship. Google is the default search engine because Google paid Mozilla a fortune.
How do their forums not support this? If it's just that "it sure looks like everyone's raging about this!" Bear in mind that social media is very bubbly, it focuses and amplifies whatever voices are the loudest and angriest. Look around one of the technology communities' reactions to anything AI and you'd be hard pressed to guess that chatgpt.com is the fifth-most-visited website in existence currently.
Got any actual survey information? I wouldn't be surprised if Mozilla's got some.