Well this is a tremendous step in the wrong direction. The economic problem is the ad supported model in the first place, no matter how it’s run. This is the same thing Google does, they keep user data to themselves and sell the ad placement. So now Mozilla has the same economic incentives as Google. Unfathomably bad move.
Given the ease of implantation of end to end encryption now, it’s a reasonable assumption that anything not e2ee is being data mined. E2ee has extensive security benefits, for example even if your data is dumped the info is still useless. So, there has to be a compelling reason to not use it.