Right click either of them to get a list of sites previously visited.

You can also install this add-on: https://addons.mozilla.org/en-US/firefox/addon/backforward-dropmarker/

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

As original questioner, I knew how they work-I wanted to know WHY they weren't working.

They are now working and I just closed Firefox then re-opened it. It still is a mystery as to WHY.