This problem seems to be related to my global account when I log in as "Ineuw", regardless of the browser and OS. and on every site related to my "global" login.
When logging in with my regular account "Ineuw", when activating "Show Preview" in edit mode,
the preview frame is displayed below the text edit boxes.
Without login, or logging in with my alternate account, when activating "Show Preview" in edit mode, the preview frame is displayed above the text edit boxes.
Steps taken to rectify:
Reset Preferences to default.
Removed vector.js and .css
Cleared browser cache and site data
Logged out and deleted all wikmedia related cookies
Logged back into the account but the problem remained.
Test & sites
Browsers tested Firefox 75.0
Vivaldi 3.0.1874.32 (Stable channel) (64-bit)
Windows 10 Pro
Linuc Mint 19.3