From Vishal Gupta's tutorial.

https://www.askvg.com/disable-or-remove-sign-in-to-sync-banner-from-firefox-menu/

STEP 1: Open Mozilla Firefox and type about:config in the address bar and press Enter. It’ll show you a warning message, click on “Accept the Risk and Continue” button. It’ll open the config page

STEP 2: Now type fxaccounts inside the preference search box and look for following preference: identity.fxaccounts.enabled

STEP 3: This preference decides the availability of Firefox Account/Sync feature inside the web browser. To disable or remove the Firefox Account feature, set the identity.fxaccounts.enabled preference to False.

To change the preference value, just double-click on the preference and it’ll change its value.

STEP 4: Restart Firefox