Tried the steps out in comment 9 https://bugzilla.mozilla.org/show_bug.cgi?id=1573710#c9 - was not able to get it to reproduce. Also tried the 'Create a new profile' button on about:profiles. Clicked it around 20 times, dialogue showed up correctly each time.
Bug 1573710 Comment 20 Edit History
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Tried the steps out in comment 9 https://bugzilla.mozilla.org/show_bug.cgi?id=1573710#c9 - was not able to get it to reproduce. Also tried the 'Create a new profile' button on about:profiles. Clicked it around 20 times, dialogue showed up correctly each time. Computer I was trying on had same driver as https://bugzilla.mozilla.org/show_bug.cgi?id=1573710#c1