This is a new feature in Firefox 63, that "Default" tries to follow your Windows 10 system theme. Several users have reported problems with the colors (accent color not implemented) making it unusable in Firefox 63.0. I don't know whether that has improved in one of the subsequent minor bug fix releases, or won't be fixed until Firefox 64.

If there was something specific about the Default theme that you liked before, perhaps an add-on can emulate it? There are numerous solid, patterned, and artistic backgrounds available for the toolbar area. There's also a new test add-on to assign any colors you like.

• https://addons.mozilla.org/firefox/themes/solid/
• https://color.firefox.com/

Hello jscher2000,

Thank you so much! That's a decent feature, but there should be an easy way to disable it. However, I'm really happy with the Firefox Color add-on, it's so customizable, I'll probably use it even if the issue is fixed, so thanks again for informing me about it.