Hi

Do you have Firefox for Android set to follow your device theme?

Yes and my phone changes its theme depending on the time of day

I am guessing that you had closed and then reopened Firefox after the Android theme had changed and it picked up that change and adjusted its own theme.

This support article should help:

https://support.mozilla.org/en-US/kb/using-dark-theme-firefox-android

I already close Firefox before the theme change but it still doesn't adjust the first time I open it after the theme change.