@mcc
Every time Firefox (or Thunderbird) gets an update, I go to their What's New page to see the changes.
I saw this new "feature" and promptly disabled it. I also tooted about it, but I have few followers.
Firefox has a tiny user base by comparison to Chrome - and much of their funding comes from.....Google! They need new revenue streams or they will disappear and this new "feature" is one way of getting that.
The fact that it was made "opt-out" instead of "opt-in" is the main issue here.