Also, with mv3 extensions, the extensions that are not actively doing some work are not really "running" in the first place but are just waiting for some event to happen that they have previously told Firefox to inform them about, but there isn't any persistent execution context that is constantly running.
I believe one reason why extension support was not-fully enabled earlier was because mv2 extensions required persistent background context for each extension and that could cause issues if Android just decided to kill that process. But with mv3 the extensions are required to be able to be suspended and then woken up on demand.