That's already the case to some extent.

But if Firefox is the only browser where real adblocking happens, I imagine anti-adblocking will become less aggressive, which will be great news for Firefox users.

Even though Vivaldi is chromium-based, some site just don't function properly or even refuse to load, but working fine in chrome, so yeah it's already happened.

You say this like web developers don't have bosses.

Are there folks freelancing as web devs out there? Sure. Is that the majority of the web we use? Absolutely not.

Already happened. Same shit with people writing for IE over netscape or early Firefox

With the rise of tablets and smartphones a lot of those devs got rude awakenings. Then Apps and Chrome happened and they disappeared back up their own arses