Just came because I stumbled over the same thing, in a few different instances, over the last days.
I have the same setup (well, Firefox Nightly, but also with uMatrix) and was seeing the same errors.
For me, enabling XHR for both of the cloudfront subdomains (on this instance here) did the trick.
I’m not sure why that would be the case; uMatrix doesn’t indicate any XHR requests to those domains, but that seems more like a bug on their side, not discourse. I’m also unsure why a force-reload (Ctrl+F5) would help in this case, but it consistently does. Without allowing XHR, every page reload (not navigation, just the reloads with F5 or opening a new tab) will blank the page.
Anyway, hope this helps anyone who also stumbles over this. 