How? Their three main competitors: Chrome, Edge (if that even counts, being Chromium based), and Safari are completely free AND installed by default on their respective operating system(s). "Not being installed by default" is already a very high hurdle for mainstream acceptance. If you tack on some sort of cost, I can't see that working well for adoption.
Personally, I think they're on the right track with the VPN service. VPN services are already something people are used to paying for, and one of the biggest issues with VPN services are trying to figure out who you can trust out of all the nameless shell companies out there. Mozilla is a trusted brand.