Hello,

Sadly access to local .html files is now completely blocked by Firefox internal security for all add-ons. You can create such links in my speed dial, but Firefox won't allow you to click on them.
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-ons page.
I hope something similar will come to Firefox one day, but until then, there is nothing I can do about it.

I'm sorry for bad news...

Best regards,

Juraj Mäsiar