nightTab saves all of its Setting, Themes and Bookmarks to the browsers local storage. This means if the browser's cache is cleared manually or by another application nightTab could be reset to the default state.
Some browsers have options to clear cache and cookies when closing the application. This might also cause nightTab to reset to the default state.
When trying to work out what might be causing nightTab to reset to the default state try disabling other extensions or applications one by one to see if the problem persists.
nightTab only supports Chrome and Firefox. This means no testing or development has been conducted on browsers like Brave, Edge or Opera to make sure user settings are saved correctly. This is outside the scope of this project.
It is strongly advised that users backup nightTab settings regularly.