[mythmon_]

This is called "Flash of unstyled content", and has been a bug since 57: https://bugzilla.mozilla.org/show_bug.cgi?id=1404468 . In 57 the delay before initial drawing was removed, which mostly makes things feel faster, but sometimes makes things show up naked for a moment.

In that bug you can see there are several things that can cause this, including website issues and certain extensions.