It seems the Firefox set the following settings in
We are unable to verify if it’s set by us or Firefox itself. We need more time to investigate.
In our lab, problem goes away when we set the settings as shown in this screenshot:
lightweightThemes.SelectedThemeID = empty
lightweightThemes.usedThemes = true
To fix this on all your client machines please use Firefox
Abou:Config PAKs and set the above value as instructed. Here is just a reference screenshot: