That concern I can get.

While I don't think Mozilla is currently doing anything I'd say is super objectionable, or really Facebook-like, they could certainly move more that direction in the future, and then I'd have a problem with it.

I don't have a problem with ads as a method of funding something, as long as you can either disable/block them, or pay to have them removed, but I think they should be a last resort, not a primary source of revenue, lest Firefox turn into a browser that just crams ads in every single spot it can until the browsing experience is garbage.