With Firefox 84.0 (64-bit) and older versions, not all visited websites change color. I use standard (default) settings in Firefox. Up to now I was not logged in, i.e. I had no Firefox account. As an example, go to this website: https://www.schubert-verlag.de/aufgaben/uebungen_b1/sb1_uebungen_index.htm ... and select each link from "'''Kapitel 1'''" onwards in turn and return ("Go back one page") to the above page again. The first 20 links are correctly displayed in blue. Under "'''Kapitel 6'''" the next three links do not flip to blue when visited. The fourth link does correctly flip to blue indicating a visited link. See attached screenshot of top part of the web page. Can you reproduce the problem? I'd be happy to see this problem resolved. Thanks!