It's a difference without a distinction. They're paying to be the default search engine so that they can show Firefox users ads in their search results. Either

It's a difference without a distinction. They're paying to be the default search engine so that they can show Firefox users ads in their search results. Either way, they're a middle man selling ads and giving a small percentage of their profits to Mozilla when Mozilla helps them show more ads (mainly to avoid anti trust, mind you ).