Hi!
A browser extension named "Enhanced iFrame Protection", which has this github repo: https://github.com/odacavo/enhanced-iframe-protection, gives the user a verbose warning when detecting iframe elements.
The warning is showed to warn the user about a potential security risk. This seems to be a logical feature Mozilla should have.
If implemented, the user should have the ability to e.g. disable/enable iframes in settings in the "Privacy & Security" tab.