You can but it won’t be respected. It will continue to default to their included Noto fonts despite whatever font you select. You can test this yourself. I’m sure they do it for some “privacy reason” but if I wanted that trade off I’d simply use the Tor Browser or one of those hardened firefox profiles.

Librewolf doesn’t respect your choice in system fonts if you uncheck “Allow pages to choose their own fonts, instead of your selections above”. I don’t use it for that reason.