Firefox usually shows visited links in purple instead of blue. This assumes you allow Firefox to keep history. Are looking for a way to make the distinction between purple and blue more noticeable, or do you mean you want an indication to appear in another place?

By the way, purple and blue are set here:

"3-bar" menu button (or Tools menu) > Options > Content > "Colors" button

In the colors dialog, the link colors are set on the right side.

Also make sure that you do not run Firefox in permanent Private Browsing mode (Never remember history).

• https://support.mozilla.org/kb/Private+Browsing

• Tools > Options > Privacy > Firefox will: "Use custom settings for history"
• Deselect: [ ] "Always use Private Browsing mode"