Favicon-Customizer worked okay until today. The user interface suddenly turned black. I had about 80 favicons, wanted to add another one, but the moment I clicked 'Add', all I got was this:

I disabled all other extensions, restarted Firefox, uninstalled the extension (and lost all favicons) and reinstalled it. This time I could change 45 favicons, but once I tried to add no. 46, I got the black interface again, over and over again.
I use Firefox 65. The issue did not occur with 64, but not with 65 right away either, so I'm not sure if it has anything to do with the version number.