It seems that many of these mining extensions are obvious scams anyway, an other commenter in this thread gave this as an example: https://getcryptotab.com/en/
It's an obvious pyramid scheme. Even if you wanted to mine Monero in your web browser (not exactly the most efficient way I'd wager) why would you share the benefits with some random third party instead of directly joining an open pool?
If you want a laugh I suggest looking at their calculator that (with the default values) tells you that you're going to win $40k if you manage to enroll 10 million people underneath you. Good luck!
This particular extension is the first result when you search "bitcoin" in the Chrome Web Store, with close to 20k ratings (all absolutely genuine I'm sure).
Websites that actually use mining as an alternative to ads don't use extensions in my experience, they simply load a Javascript/wasm miner à la coinhive.