There is some degree of independence. For example, if Mozilla releases some super evil patch tomorrow, I'm pretty sure everyone would just patch it out immediately. In fact, this is what most derivatives seem to do, patch out the ad/telemetry stuff.

But yeah, these are all modified Firefox browsers. Hopefully, nobody was thinking these were unique, new, browsers.

No, being a fork doesn't mean it's the same browser.

There is currently no comptetive engine that isn’t owned/developed by a big company. Ladybird is slowly getting there, but it’s gonna take a while. Until then gecko engine is OK for now. It’s all opensource so we know when that’s no longer the case.