Another good example of a freemium browser extension is Grammarly. Browser extension certainly can be monetized in an ethical way. (Note: I never used Grammarly myself, just seen the many ads)
There's also:
"You can remove your Personal Data from Grammarly at any time by deleting your account as described above. However, we may keep some of your Personal Data for as long as reasonably necessary for our legitimate business interests"