Firefox Monitor and Firefox Relay are good ideas for subscription services that may be useful to users and hopefully get revenue.
When I looked closely at Firefox Relay, the email feature was redundant because I also have a service which does this, and the phone feature isn't available yet. Looking at Firefox Monitor and the list of companies/brokers it monitors, these appear focused on the US which isn't where I live.
I hope they can get revenue by promoting these services and making them useful for more people. This would be better than showing ads. I'd pay for a useful service, not to have an-free experience for something which is freely available with ads.