A: the main problem with ads is tracking and privacy invasion
B: the main problem with ads is manipulation and seizure of my attention
If you only care about A, you might like this approach, as in principle if it works and becomes standard, then the pressure from the ad industry to track everything will be easier to resist, as 1) they will have less incentive, and 2) the argument that tracking is essential is undermined, so it may be possible eventually to ban it.
Of course, that assumes that you trust that this is better for your privacy than tracking. After all, it does feel a bit like tracking... If it's done properly then your individual data is not sprayed to a thousand dodgy ad brokers, but only to one company who tells advertisers not about you personally, but just whether their ad is working.
The question is, does it work? Are they doing it properly? Do they have the correct incentives to keep doing it properly in the long term? Can advertisers just undermine it by giving everyone's ads a different ID? Also, your threat model may include that this aggregator company is hacked, or that the government secretly forces it to share the data with them.
Nevertheless, I think that Mozilla probably genuinely think this is better for privacy. And there is a case that there is.
A big issue however its that at present the constituency for Firefox includes people who care about B. This doesn't undermine that directly, but it does mean that Mozilla have an incentive not to care about it.