So was the menu color a default Firefox setting or custom theme not provide on default Firefox? So that others can verify what is happening or where did you get the theme or menu theme color from.

It is possible you selected one of the Colorways themes that are included in Firefox.

You can find the built-in themes (light, dark, system) and further down the Colorways themes included in Firefox on the "Add-ons and Themes" page (Ctrl+Shift+A; Themes tab) and go back to the theme you were using previously like the System theme.

• https://support.mozilla.org/en-US/kb/use-themes-change-look-of-firefox
• https://support.mozilla.org/en-US/kb/personalize-firefox-colorways