Not entirely sure I understand your opposition to a great security feature like Firefox Monitor, but you can disable notifications about your accounts by unchecking the Show alerts about passwords for breached websites option in the Firefox options page.

Hope this helps.

Disabling the notification is not the same as disabling it completely.

The idea of it being great or needed is debatable depending on who you are. Sure some people may find it very useful. The there are those that won't. Have an option to completely disable it will go long way with privacy advocates.

You can unsubscribe from Firefox Monitor. This should stop Firefox from communicating with the database API that looks if your email addresses and passwords have been leaked. For instructions, see: https://support.mozilla.org/en-US/kb/firefox-monitor-faq#w_how-do-i-unsubscribe-from-firefox-monitor-alerts

Also, here's an article that explains how the communication between Firefox and the database is done. You'll see that no plaintext information is sent. Everything is anonymized before. https://blog.mozilla.org/security/2018/06/25/scanning-breached-accounts-k-anonymity/

As a footnote to the above:

Even if your Firefox is not set up to proactively notify you, and even if you unsubscribe from email notifications, the Firefox Monitor website (https://monitor.firefox.com/) and the website of Mozilla's partner for breach data (https://haveibeenpwned.com/) will continue to provide search features you can use when you get the itch to find out about breaches affecting your email address (and potentially other data).