Firefox can’t find the file at jar:file:///usr/lib/firefox/omni.ja!/chrome/toolkit/content/mozapps/extensions/null.

I had been using FF prior to installing a single boot Linux Mint 19.2 Cinnamon Tina. I went from Linux Lite to Mint. When I opened the freshly installed FF and pressed "sync now" it restored all of my add-ons. All of the files were there. All thumbnails too. Now, when I go to Customize and click on the themes menu, I do not see any thumbnails of the downloaded themes except for the 3 default themes that come with FF. I also am unable to mouse over those 3 themes and have them display what they look like real-time. When I click on manage themes (and I have many), there are no thumbnails. When I right click and ask to view the image it gives me the following error: Firefox can’t find the file at jar:file:///usr/lib/firefox/omni.ja!/chrome/toolkit/content/mozapps/extensions/null. Everything was functioning properly until yesterday. So a total of 3 days. How can I resolve this?