I tested the basic “Borderify” add-on example in Firefox 56.0.2. I checked “Enable add-on debugging” in “about:debugging” and then I clicked Debug for the Borderify extension. I clicked “OK” to the prompt for remote debugging. The Developer Tools window that comes up shows no sources in the debugger. Shouldn’t it show “borderify.js” and allow me to debug that?
If borderify.js is a content script, which I think it is, the no.
Content scripts are executed in the content process(es), the debugger is attached to the extension process. You should be able to see the borderify script in the page debugger on all pages where is is applied.