Thanks for the help

I just have the link colors set in Tools -> Settings -> General and clicking the Colors button to set different colors for links. Screenshot: https://i.imgur.com/l8rLcX0.png

Granted, Firefox's History needs to be enabled for it to remember what you've visited. I haven't tried that linked extension.

Note that you likely need to add the !important flag in your userContent.css to override the color specified by a website.

• a:visited {color: FF0000 !important;}

First, Google conducts a lot of user interface experiments. For example, you might have the mobile layout which styles unvisited and visited links the same. You should be able to override that, but you also could try escaping the experiment by clearing the cache and deleting your Google cookies. Details below if you want to try it.

Second, I'm pretty sure colors written in hexadecimal notation require a hash/pound symbol. Alternately, you can use a color keyword.

a:visited {color: #f00 !important;}
a:visited {color: red !important;}

(1) Clear Firefox's Cache

See: How to clear the Firefox cache

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that.

In the dialog that opens, you will see one or more matches to the current address so you can remove the site's cookies individually without affecting other sites.

Then try loading Google in a new tab. Does that help?