This only happens if the extension puts their `moz-extension://` links into the DOM. It's different to chrome case where extensions can be detected regardless of being activated on that site or not.
As I understand it, an extension could also leak its links via its own backend, e.g. to advertisers, who could then detect it even though no user-observable DOM modification is happening.
Much better than static global IDs, but still not ideal.
Yeah, anything happening in backend depends totally on the extensions. Unless I need something, I rarely use extensions that are closed-source or open-source but has some sending data in their features.