Re: Retrieving a webpages shows a blank screen when a 4xx HTTP status is returned. Not helpfull.

Dave,

> What Fx version was current on the v1 release date?

I have no idea.

> Do you get any console errors?

After clearing the console and roloading the temporary add-on I see one 
(after the unload and a "browser_style" not being specified warning 
message): "ExtensionChild.jsm:901"   It points to the 
"document.location.replace(url)" line in the below :
- - - - - - - - - - - - - - -
        if (url) {
          // TODO(robwu): Remove this check. It is only here because the 
popup
          // implementation does not always load a URL at the 
initialization,
          // and the logic is too complex to fix at once.
          let {document} = this.global.content;
          this.initialDocuments.add(document);
          document.location.replace(url);
        }
- - - - - - - - - - - - - - -

> You could check on MDN when the APIs were introduced

That could be a next step, after I figure out how to debug a "background" 
script. Without it I'm just stumbling in the dark.

Further stuff tried:

I've installed FF v52.5.0 on a spare machine, and saw the temporary 
installed plugin work.   The HTTP status code is displayed in a small 
rectangle over the add-ons icon.

Also, the Ctrl-shift-J "browser Console" showed the "console.log" messages I 
had placed at the top and bottom of the background script (something that I 
didn't see on the origional machine).

Something to recon with is that the "Developer Tools" -> "Debugging" window 
shows the loaded background script only once: when first loading the add-on. 
A "about:debugging" -> "Reload" causes it to disappear.

tl;dr:
It looks that that my FF on the origional machine has developped a problem. 
With all the "about:config" settings e.a. I've changed I've got to think 
about if I want to go as far as to re-install it ...

Regards,
Rudy Wieser

P.s.
FF 52 is connecting to a truckload of URLs (Mozilla and others) just after 
having been installed.  Without having been asked permission for any of them 
...