It looks like your display switched from using page colors to system colors. You should keep the default setting of using page colors (I suggest Override... "Never" -- see the attached screenshot).

If the page has its own link colors you need to override, then you could use an add-on for that. The add-on will modify the style rules in the page, so it can only work if you have Firefox use the page's colors rather than forcing default colors on the Options page.

Alternatively you can use code in userContent.css to specify link colors.
With userContent.css you can possibly override the colors only for specific websites.

See also:

• https://www.userchrome.org/what-is-userchrome-css.html
• https://www.userchrome.org/how-create-userchrome-css.html
• https://www.userchrome.org/firefox-changes-userchrome-css.html

visited-color-picker https://addons.mozilla.org/firefox/addon/visited-color-picker/

I did not have "use system colors" checked. If I set it to never, it never changed the color. I had tried a different color changer add-on but it had the same problem as FF built-in option. I tried visited-color-picker suggested by cor-el and it works like I want it to. Thanks.