This used to work really well. With firefox 83.0 on OSX 14.1 it cannot render a local file. I wasted way too many hours trying all the work-arounds described in the github page and on superuser. None of them worked.
I’m sorry to hear about your experience. Local files are tough to handle between the mime types, operating systems, Firefox… Hopefully the support will improve with future Firefox versions, but I’m afraid there’s nothing more the add-on could be doing.
