Type about:addons<enter> in the address box to open the Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

On the left side of the page, select Appearance or Themes. Then select Default or System theme — auto. Now restart Firefox.

Thanks for the response. While that changed the color to bright white, it also set everything else to high-contrast. That includes all fonts and other images. I guess I should have been more specific in my request. I want Firefox to go back to the way it looked before the update. I didn't know I had more things to "fix".

Check this link: https://support.mozilla.org/en-US/search?q=high-contrast

What theme were you using? What do you mean by 'header'?

I *was* using the default, until it changed via update. When it changed, it changed to something I can only kindly refer to as dog vomit. When I tried customizing as suggested, the entire page is bright white, with high visibility black text. The header is the part at the top with addresses, shortcuts, and toolbar. I'm wondering if refreshing Firefox will do anything, other than losing what I still have, and starting completely over.

Try the normal fixes first.

Refreshing Firefox {web link} is a LAST RESORT ! !

Refreshing Firefox -- easily fix most problems

If you're having problems with Firefox, refreshing it can help. The refresh feature fixes many issues by restoring Firefox to its factory default state while saving your essential information like bookmarks and open tabs. Everything else, like user settings and add-ons, are removed. Note: Refresh saves your old profile to the desktop in a folder called Old Firefox.

It seems you may have got one of the Colorways/Colourways themes. You could choose another of those themes, some other theme or install a theme. By 'default' you could mean any number of themes. What colour do you want the toolbars to be? Choosing a different theme takes effect immediately.

Since FF96, there's a complication. If your tab text is light, Mozilla has decided that you must have a dark theme and vice versa. There are ways of dealing with that.