Can you reproduce the problem if you set the pref `extensions.formautofill.available` to "off" and then restart Fx? We do initialization of Form Autofill on the first click inside a relevant field so it's possible we're interfering with the site's event.
Bug 1556501 Comment 2 Edit History
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Can you reproduce the problem if you set the pref `extensions.formautofill.available` to "off" and then restart Fx? We do [initialization of Form Autofill on the first click inside a relevant field](https://searchfox.org/mozilla-central/rev/f8b11433159cbc9cc80500b3e579d767473fa539/browser/extensions/formautofill/content/FormAutofillFrameScript.js#78-100) so it's possible we're interfering with the site's event.