When I edit emails and landing pages using Chrome, a line gets added to the end of the html: "<div id="i4c-dialogs-container"></div>." This does not happen with Firefox.
This might have something to do with a plugin that you've got installed in Chrome but not Firefox? I didn't find much on the internet but my hunch is that it's some shoddy script or bad plug-in that's not made to consider an editable interface like Marketo.
I found an (unanswered) article on the internet from someone running into a similar issue using a different editor interface: https://github.com/ckeditor/ckeditor4/issues/3683 - maybe this is a similar issue to what you're seeing?
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