I am unclear on the issue, as Thunderbird has always shown links in color, although the color is not part of the embedded HTML. Sharing more information may help highlight the problem. Thank you.

The question is what is controlling the highlighting and how do I change it?

check settings>general and click the colors button in the fonts&colors section.

Additional comment: Recipients see link colors as defined in their own email client. The color you see is not communicated in the message.

The real answer was to go to Settings/Privacy and Security/ and uncheck the box for "remember websites I have visited."

I forgot that this was a thing, along with cookies.

I am forced to assume that unchecking the box deletes the history, because unlike every browser I use there is no "delete site data/delete history" option - just the option to delete cookies.