Mullvad also states that it disabled the Firefox password storage feature, because it's supposedly insecure. But the articles supporting this view (i read) seem

Mullvad also states that it disabled the Firefox password storage feature, because it's supposedly insecure. But the articles supporting this view (i read) seem to be written by third-party password storage friends. Their arguments are weak (like "some managers used to do bla bla, which was insecure") and don't apply to Firefox. Is there a strong argument specifically against Firefox passwords and password sync ?