Since it had been awhile, I tried to create a new Firefox account with an alternate e-mail of mine...and found that there is an opt-in check mark for newsletters and IT'S UNCHECKED BY DEFAULT. Since I can't embed images here, have a Send link to a screenshot:
So it is starting to seem like you are intentionally smearing Mozilla for a fake problem.
Not to mention you are conflating marketing spam with privacy issues.
Edit: I made the send link valid for 100 downloads or 7 days which is the max...but if you click the link and it doesn't work just know that it's because it has hit the limit, not that Send is unreliable.
From the blog post linked in this topic.
Edit (to address your edit about marketing emails and privacy):
Respectfully, I disagree. Privacy issues center around the invasion, and in extreme cases outright elimination, of privacy of individuals. This is when entities use (your) data to identify you even when you are trying to stay anonymous. When it becomes impossible for you to have privacy.
To be honest, this central issue has a fairly vast umbrella...but no, taking an e-mail (without a name even) and placing it on a marketing campaign list with explicit user consent via a true opt-in does not fall under that umbrella in my opinion.
Also, to get to the second point: you didn't click the opt-in checkbox and still got marketing e-mails?
(Just signed up for a new account, will do that with a GDPR template if I receive unwanted e-mails)