Opera regularly sponsors YouTube videos, Firefox doesn’t. Marketing works.
Like 1M/yr spent on sponsoring 1000 videos by top tech youtubers is nothing when they have 600M/yr revenue and it would bring in a massive audience.
It's as if they're content with coasting and/or chasing even lower single digit market share?
If I ever see a FireFox marketing campaign on YouTube I will worry about them.
The best “marketing” they can do in my opinion is something they already do: MDN.