[PLUG] Firefox add-ons dropped, can't re-install

[Rich Shepard]

Running firefox-60.6esr here. Since yesterday I've lost ad-block plus,
ghostery, and a download manager with firefox telling me they're not
supported in firefox. Yet, when I select the Add-ons item from the Tools
menu and click the link for more add-ons -> ad blockers both show up.

Trying to install them fails.

In all the years I've used firefox this has not happened. Has anyone else
experienced this?

Rich