Obtaining URL of page with basic authentication in Firefox

I have a page which requires Basic Authentication – for example https://authenticationtest.com/HTTPAuth/. What I want is to obtain the URL of this page before the users enter their credentials.

In Chrome I am able to do this via both the extension (chrome.tabs.query -> tabs[0].url) and via pure javascript (window.location). In Firefox however both information displays “about:newtab”.

Is there something I’m missing when it comes to pages with Basic Authentication in firefox?

I’ve tried to search the web for a related problem but I couldn’t find any.

I’m expecting to obtain the URL (in the example given authenticationtest.com/HTTPAuth) in some way just like in the case of Chrome.