For those curious, Gumroad does offer support for generating license keys which they state "are primarily used for creators selling software."[0] From my own experience implementing this and having used other extensions that also relied on this system, I think it works well for bringing monetization to browser addons/extensions.
EDIT: just wanted to add that you don't need to distribute your extension through Gumroad (though you can). You just make it so that anything behind a paywall requires adding the license key which is generated by Gumroad upon purchase and later verified via their licenses API.