There are no errors, Firefox just won’t react on clicking on elements pointing to local files.
For example:
<a id="dial_13" href="file:///C:/Users/Juraj/Desktop/simple.html">Click me</a>
will not open file “simple.html” from my desktop (if I copy the ‘href’ value into addressbar I can view the html file).
I understand the security issue here but I believe the add-ons deserves this functionality.
There is also some article here about accessing files https://wiki.mozilla.org/WebExtensions/Filesystem