A search engine frontend produces content in response to a user query, not proactively. Even before the first query I get an option to choose. It's not the same thing. It's more on the content conduit side than operator-run showcase.
Has the idea that Mozilla won't survive without advertising income been substantiated somewhere? (This ad was unpaid, though.)
Installation of the addon was not opt-in. Receiving any form of promotional content was opt-in via about:config. There is an open conduit; anybody else could offer an addon.
So I agree that it was not neutral, because there was more of a barrier to other advertisers than to Mr. Roboto. The right thing would be to have hosted the addon on addons.mozilla.org, same as for anyone else.
But it's not a dramatically uneven playing field, given that an open conduit does exist and the reception of any actual advertising was opt-in. (I'm not saying it was ok; it wasn't. But it's not as black and white as you are saying.)
Has the idea that Mozilla won't survive without advertising income been substantiated somewhere? (This ad was unpaid, though.)