I recently attempted to log into a new Firefox instance with my Firefox account. As expected, I received an email with a verification code from accounts@firefox.com to my gmail account as part of the login flow.
This email was flagged by gmail. Above the body of the message, gmail inserted a scary looking red banner with the following text:
This message seems dangerous Similar messages were used to steal people's personal information. Avoid clicking links, downloading attachments, or replying with personal information.
A link with the text "Looks safe" was presented (which I did click).
I made sure to check all the headers in the original message, and everything looked correct.