Hello,

Sadly access to local files through "file://" protocol is now completely blocked by Firefox internal security for all add-ons.
More boring info here: https://wiki.mozilla.org/WebExtensions/Filesystem

Situation in Chrome browser is very similar but Chrome added option (checkbox) to enable access to local files for specific add-on on their add-on page. I hope something similar will come to Firefox one day, but until then, there is nothing I can do about it.
You can also downgrade to Firefox ESR where old add-ons still works :)

Good point with the background, I'm more focusing on features than look (which may not be best for business :)). But I'm planning some exciting upgrades that will include new background for every day from Bing service :)

Anyway I'm sorry for the bad news.

Have a nice day :)

Best regards,

Juraj Mäsiar