You need to change the theme to one you like. Open Add-ons and Themes (in the menus under Tools or More tools or use the shortcut ctl+shft+a) and click on Themes in the left panel.

Disable the theme which is Enabled and enable the theme you want to use. If you want to try a theme which isn't there, go to https://addons.mozilla.org/firefox/themes/

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).

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

I'm not talking about themes or light/dark systems. The update included a choice of colour for the main frames. How do I change that???

You can choose another theme if you do not like the color of the current theme.

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

What theme did you have in the previous Firefox version ?

What do you mean by "main frames"?