Thanks for sharing, @gantnerer!
It’s a little tricky to look through the historic Firefox code base. This is what I was able to find regarding the colors used in the pre-89 default themes. I haven’t tested them so they may not be totally accurate.
Default light theme
"tab_background_text": "rgb(24, 25, 26)",
"icons": "rgb(24, 25, 26, 0.7)",
"frame": "#E3E4E6",
"popup": "#fff",
"popup_text": "#0c0c0d",
"popup_border": "#ccc",
"tab_line": "#0a84ff",
"toolbar": "#f5f6f7",
"toolbar_bottom_separator": "#ccc",
"toolbar_field": "#fff",
"toolbar_field_text": "rgb(24, 25, 26)",
"toolbar_field_border": "#ccc",
"ntp_background": "#F9F9FA",
"ntp_text": "rgb(12, 12, 13)"
Default dark theme
"tab_background_text": "rgb(249, 249, 250)",
"icons": "rgb(249, 249, 250, 0.7)",
"frame": "hsl(240, 5%, 5%)",
"popup": "#4a4a4f",
"popup_text": "rgb(249, 249, 250)",
"popup_border": "#27272b",
"tab_line": "#0a84ff",
"toolbar": "hsl(240, 1%, 20%)",
"toolbar_bottom_separator": "hsl(240, 5%, 5%)",
"toolbar_field": "rgb(71, 71, 73)",
"toolbar_field_border": "rgba(249, 249, 250, 0.2)",
"toolbar_field_separator": "#5F6670",
"toolbar_field_text": "rgb(249, 249, 250)",
"ntp_background": "#2A2A2E",
"ntp_text": "rgb(249, 249, 250)",
"sidebar": "#38383D",
"sidebar_text": "rgb(249, 249, 250)",
"sidebar_border": "rgba(255, 255, 255, 0.1)"
If you want to create a theme using these colors, you will either need to code the theme yourself and submit it to addons.mozilla.org or create it using Firefox Color. (Quick note about Firefox Color – if you design it and then keep the FC extension installed, the theme will be applied. If you remove the extension, the theme will go away.)
Some of these theme properties are not supported at all in Firefox 89, so even if you assign a color to background tab separators, they won’t show up in the browser. You can check this page to see what properties are supported and what part of the browser they correspond to.