Visited link color change feature randomly quits working

I have several saved searches on Craigslist bookmarked, and I check ads every day. As I click on a link in my search category, that link changes color every time. But randomly, when I go back to that same search, the most recently clicked-on items have reverted to the original color, as if I had not visited them. All the other previously visited items below those have kept their “visited” color — only the most recently visited are affected. The workaround for this is to close and re-start Firefox, and then everything works fine for a while. I put my notebook into hibernation overnight, and sometimes in the morning it’s fine, and sometimes I have to re-start to get newly visited items to stay marked as visited. Sometimes, after working fine in the morning I’ll come back in the afternoon to find the problem occurring again. I’m using 64 bit FF 68.0.2 now, but this has been happening for months through previous versions as well. Suggestions would be welcome.