Hmm, I see what you mean. In the new Contrast Control section, link colors now can only be customized as part of High Contrast mode.

I don't see an immediate workaround using built-in features, but I'm sure others will ask about this so I'll keep an eye on it.

Meanwhile, you could consider using an add-on that lets you apply your own styles to web pages. Here's how that would work:

(1) Install the Stylus extension: https://addons.mozilla.org/firefox/addon/styl-us/

This add-on lets you apply custom style rule overrides to web pages.

(2) Find the Stylus "S" button on the toolbar or Extensions button drop-down list, click that button, then click Manage to open a new tab with the Stylus dashboard.

(3) In the left column, look for and hover a heavy + button. If the tooltip says "Write new style" go ahead and click.

In the first field at the left, enter a name like Visited Brown and on the right side, paste this code:

a:link {
  color: blue !important;
  text-decoration-color: blue !important;
}
a:visited {
  color: brown !important;
  text-decoration-color: brown !important;
}
 

Then click Save. You can use a page with links to test the color. To fine-tune, back in the Stylus editor, click the little colored square to pull up a color chooser (example screenshot attached).

Stylus works somewhat but the selected color for the links differs by web page--the color intensity can be markedly different. Wonder why. But thanks.

The 138.0 update broke the addon "visited-color-picker" which previously worked perfectly and the color was identical on every page. It also broke "visitedlinkenabler" which wasn't quite as good but still did the job.

So Stylus will work for me as a temporary work-around, but the developers need to bring back the older color picker option.