Speaking as someone who has a 30 or more tabs open at any given time -- often from the same site -- this "feature" is making my browsing experience completely unbearable. The developers should at least have the decency to give the end user the ability to turn this abysmal nonsense off.
Your primary goal is to attract new users and retain current ones, and new features are nice (and necessary) in a browser market filled with competitors. I get it. But adding new features without providing a way to opt in or out is just bad form. Not all of us want new bells and whistles forced down our throats, but here we are.
Granted, your "average" user won't mind, but you risk alienating users who would recommend Opera to others. I'm one of those users, and I'll steer those in my sphere of influence away from Opera because you're reinforcing my belief that the end user's opinion doesn't factor into your decision-making process. Firefox lost users by doing whatever they wanted without regard to the interest or opinion of their user base, and you seem to be following suit. Being comparable to Firefox hasn't been a positive thing in over a decade, and you'd be wise to correct the errors that have led us here.