This workaround works perfectly. I experienced the issue with the Disconnect extension (in any browser) which is the database used by FF to protect from tracking.
But it seems that there's still a problem over https.
I have 2 websites with the same types of forms and the one who is secured doesn't display the forms and generate errors like : ReferenceError: MktoForms2 is not defined.
These errors appear only when I replace the loading URL by the landing page URL.