I'm using macOS 11.0.1, with Automatic theme-switching enabled in System Preferences. In the evening, when the theme switches to Dark mode, Firefox's theme changes to Dark also. If I don't shut down my Mac, Firefox's theme will be Light in the morning, which is normal. However, if I shutdown my Mac completely in the evening and turn it back on the next morning, Firefox's theme keeps the last state (Dark) also my system's theme changes to Light. The only workaround is to change my Mac's theme to Dark/Light only, and change it back to Auto again.