I run the Web Clipper in Firefox v. 83/Windows 10. It does clip and transfer web pages as it should. However, whenever I enter the about:addons page the N button on the search bar gets grayed out or vanishes completely, see the attachment.

The add-ons manager complains that a relevant file is not found at moz-extension://... , as shown in the attachment.

After reloading the Web Clipper a couple of times I realized that the Web Clipper button can be reactivated simply by disabling and enabling the add-on, or by toggling the "Run in Private Windows" switch. The Web Clipper really starts working again.

Seems odd. Don't know whether this is a FF or Nimbus bug.