google search does not retain "visited" color....
Wšě wotmołwy (1)
There may be a few different contributing factors:
(1) Google changed from using the real link for the result title to using a coded link which doesn't match up with the real link in your history. If you hover over the links, they have a Google URL with "goto" and then a code.
I'm not aware of an official setting to turn this off, but the multi-purpose uBlock Origin extension restores the original links somehow, if you want to try that: https://addons.mozilla.org/firefox/addon/ublock-origin/
(2) Firefox should use the visited color for links that match a URL in your history, but websites can override Firefox's colors.
From time to time, Google has done this in layout "experiments" conducted on users. Even though I'm not seeing a change, you might be seeing it if you are in the test group.