Etsy page message :Etsy is no longer supporting older versions of your web browser in order to ensure that user data remains secure.
Why can't update browser?
MacBook Pro 15″, macOS 10.13
If you are using Safari you can only update this by updating your System, ideally to the latest one (Monterey) though Catalina may be acceptable (Etsy don't seem to specify an actual rquirement).
However the latest version of Firefox should run on 10.13 so a simpler solution would be to install that.