Unfortunately, with 12.x it still cannot be disabled.
You can read the long-standing argument here on bugzilla about this very problem. Log in and support him!
I have found complaints about this post for at least six years. This is regularly reported on bugzilla and other Mozilla support sites and is a popular issue in many other support forums. According to most Moz developers, this is a critical security feature that should not be disabled. Personally, I find it very annoying, I hit MSDN through Google search queries all day, and I have to click on this stupid warning every time. If you see this a lot, it quickly becomes one of those warnings that you automatically ignore, which makes the โmandatory securityโ argument quite unconvincing.
McGuireV10
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