You may restrict updates in policies.json[1][2]:

  {
    "policies": {
      "AppUpdateURL": "http://127.0.0.1/",
      "DisableAppUpdate": true,
      "DisableFirefoxStudies": true,
      "DisableSystemAddonUpdate": true,
      "DontCheckDefaultBrowser": true,
      "ExtensionUpdate": false,
    }
  }

File should be located in $FIREFOX_DISTRIBUTION_DIR/distribution/

$FIREFOX_DISTRIBUTION_DIR is the installation directory for firefox, which (for default setup) probably will be printed by this:

  sed -nE 's/^\s*exec (.*\/)firefox .*/\1/p' $(which firefox) # and it's probably /usr/lib/firefox

[1] https://firefox-admin-docs.mozilla.org/guides/policies-confi...

[2] https://firefox-admin-docs.mozilla.org/reference/policies/

Even better, I switched to the esr release. It gets updates every week or two, but only for security issues. Otherwise the browser has a stable feature set and no new features.