Simply announce the intention before and with the update, be clear about it and how to truly/fully enable and disable it. Doesn't even have to be a popup.

This and many of their recent interactions have avoided that transparency and been very disengenuous with their communications before and after the fact.

When you start to think that you have to be sneaky to get ads in front of users, or to monetize at all then you are already thinking of it as 'us vs. the users' and that's quite a bad position to be in. Especially for a company who has been banking on the user goodwill market lately.

How they're going about this paints a very conflicting picture of Mozilla. The organization seems bipolar at best.