Even if you just blocked trackers (as does ETP Strict Mode), many ads get blocked because those ads bundle tracking code within them. This, again, would cause there to be little to no incentive for most website owners to support Firefox.
What Mozilla is currently doing makes sense. They are being lax on the standard setting so that websites can still make ad revenue and have an incentive to support the browser.
If Firefox had a lot more marketshare, it might have been possible that this could slide.