Whether particular persona or all the persona themes.

Remove and reinstall the persona

In order to switch to another persona that you had previously installed, do the following:

1. Go to Tools and click Add-ons (or click the Firefox button at the top left and click Add-ons).
2. Switch to the Appearance tab. You should find your previously used personas in the list.
3. Find the one you want to use and click the Enable button.

To uninstall the other ones you can click the Remove button on them and they'll be uninstalled instantly.

Make sure that you do not run Firefox in (permanent) Private Browsing mode.

• https://support.mozilla.org/kb/Private+Browsing

• Firefox > Preferences > Privacy > Firefox will: "Use custom settings for history"
• Deselect: [ ] "Always use private browsing mode"

If you switch to Private Browsing then a special built-in persona is applied to disable the theme and this makes it impossible to use other personas.

Excellent! It solved the problem right away! Thank you very very very much!