Comment by kasabali

I use a 2 step process for this.

1. I installed an extension (can't remember the name) that makes visited links (from history) purple (like the old times) so that I won't reopen a duplicate tab

2. After reading a page I add them to bookmarks in a specific (have read) folder so that they're marked by the star in Firefox URL bar and I can easily check if I've already read them just by looking