Yes, it is accepted. What is so wrong about advertising? Why is it bad that someone making a product wants to market it? There is absolutely nothing wrong with it. The problem is how the ad-tech industry deals with advertising. There is no regard for privacy and no control for the consumers. It's become a shady practice because of lack of privacy, transparency and lack of regard for the end user.
Mozilla is clearly saying these things are important to them and will be central to their mission so what's the problem here? It's not like they'll inject ads from Google or Facebook.
Instead of dissing at their attempts to make themselves self-sustainable, we should be supporting them by giving them a chance. May be they'll do ads right, may be they'll fail but one thing I'm sure of is that if they fail to make it work without compromising the user, they'll just give it up.
Mozilla is our biggest if not the only hope of an open and healthy web. I don't mind supporting them by seeing a couple of ads every now and then as long as I'm in control and they respect my privacy.