Appearance > Customize shows this text instead of the theme page.
"Blocked by Content Security Policy
An error occurred during a connection to mysite_staging.wpmudev.host.
Firefox prevented this page from loading in this way because the page has a content security policy that disallows it."
This occurs with all themes that I've tried. Google returns many hits on this related to cookies, etc. Sure, I can clear cookies for this site. The challenge isn't to fix the immediate problem. I'm passing along a note that this is happening in this environment, and I've never seen it elsewhere. I have not yet tried to use Chrome for this site but will probably do so just to get around this issue.
I'll open the site for Support. Feel free to check it out … with Firefox.