It may be hard to believe, but including a built in popup blocker was revolutionary. It was called "irresponsible" by advertisers, and forced everyone else to change. Could you ever imagine Mozilla undermining the big players like that today?
That is not at all the same as when they first did it. When they did it, popups (that is, ads that would open a new browser window) were a staple of online advertizing, and they disrupted it.
Google is also doing the same thing, including punishing websites that use abusive interstitials and popups[1].
It would not be all that disruptive at all, in fact it is actually helping Google and the big platforms by forcing ads to be more palatable.
I doubt that any of Mozilla's underwriters have a problem with this.
Something that would be disruptive would be if Firefox bundled uBlock by default, which Google is trying to prevent users of Chrome from using[2]. If Google wanted to, they have the means to remove and blacklist the extension from most users computers immediately, thanks to their updating mechanism. They could even push changes to Chrome that break the extension or any like it, immediately. Frankly I think the only thing that acts as a counter balance is Firefox, but they making themselves less relevant by the day.
I mean yeah... they're doing that right now. https://github.com/ehsan/popup-reporter
Edit: and Tracking Protection already launched https://support.mozilla.org/en-US/kb/enhanced-tracking-prote...