Are the theme a default from firefox? If not you might have to go to the site where you got them and see if there are any problems with them.

What theme are you talking about? Are you using sync?

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

What theme is this about ? Are you talking about a lightweight theme from the add-ons website ?

Themes are stored as a pref in prefs.js in the profile folder. Do you otherwise have a problem with keeping preference settings?

There is software like Advanced SystemCare with its Surfing Protection feature that can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

• https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save