The Google search bar that appears in Firefox when you install it, with a Google icon, is effectively an advertisement paid for with Google's money. The fact that it's an effective advertisement (and most people use that search bar) doesn't make it not an advertisement.
Note that for example the default search provider has been different in other countries at times based on other advertising deals Mozilla made - not just Google. It's easier to view it as not-an-ad when it's a search provider you use; less so than when it is a random provider that bid the highest amount.
Note that for example the default search provider has been different in other countries at times based on other advertising deals Mozilla made - not just Google. It's easier to view it as not-an-ad when it's a search provider you use; less so than when it is a random provider that bid the highest amount.