Ew. Any browser pulling that shit with me goes straight to the discard pile. I get that enough from websites; I should never have to see that from my own browser.
Really sad to see that from Firefox, though. I'd have expected it from Edge or even Chrome; suggests that Mozilla is getting desperate.
Possible solution as stated on https://old.reddit.com/r/firefox/comments/13rnt88/why_is_fir...
In about:config set browser.vpn_promo.enabled to false.