This seems to have come in with a recent Facebook Container release. Facebook Container now appears to make the same allegation about any site with a tag <input type="email">.

See jscher2000's workaround here: https://support.mozilla.org/en-US/questions/1345601

And the issue on Github: https://github.com/mozilla/contain-facebook/issues/783

Another workaround is to add to your input element:

   class="fbc-has-badge"

This seems to make the extension think that it has already dealt with your input field, and leave it alone.