The benefit of the doubt logic here Is as follows:

Mozilla thinks that the ad supported model of the web is essential to continuing to function.

Mozilla knows that the vast majority of installs have an adblocker installed.

In order to keep the ecosystem healthy they’re exploring ethical advertising options.

Counterpoint:

The vast majority of their income comes from the largest advertising company in the world - alphabet.

They need to be seen to at least make some efforts to not murder the advertising model through their mere existence while not pissing off the community too much.