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.

I have this problem too. It's persistent tru years as well.

My phone changes light/dark theme tru day. If Firefox is opened in background, when focusing back to it stays in theme witch was last time.

To reproduce problem.

Set Firefox to follow device theme Open Firefox -> go on home screen -> change device theme -> go back to Firefox