More obvious when a site has a lot of content or on a slow connex, the site preview loading spinner styling is odd on Firefox. Note that the spinner for editor/post preview is correct:
https://cloudup.com/cFEq4nWY9Ku
This issue also occurs on the stats page when switching between Insights/daily/weekly/monthly stats views.

Confirmed it happens in Firefox, but not in Chrome.
Reported in 744627-f
Unable to reproduce this anymore (FF 48.0.2, macOS):

Closing for now. Feel free to re-open.
Still happening for me, FF 49.0.1, Mac OS X
@lsinger See if you can repro if you simulate a slow connection, say under 1mbps

ACK I now was able to reproduce with a slow connection.
Removing GuidedTours and NUX labels and adding Components label though, since this seems to be an issue with the Spinner component.
I can not re-create this in FF 50.0.2, an easy way to test is to open up http://calypso.dev:3000/devdocs/design/spinner which is the component in question. Working fine for me.
It does appear to be working on the stats page for me now as well, despite my slow connection :)