Hi nickell1, the site uses Facebook for its login system. If the site cannot access facebook.net, then the dialog does not appear (apparently the developers didn't account for that possibility). There are a few possible reasons that might occur, in particular, these come to mind:

(1) Firefox's Content Blocking (Tracking Protection) feature will block that. In that case, you'll notice a "shield" icon toward the left end of the address bar you can use to make an exception.

(2) The Facebook Container extension likely will block it because it enforces isolation of Facebook from other sites.

(3) Anti-tracking and ad blocking extensions such as Adblock Plus, Blur, Disconnect, Ghostery, NoScript, Privacy Badger, uBlock Origin or uMatrix could also be an issue. Each one you have installed should provide a toolbar button to manage blocked content in a page. There may or may not be a number on the icon indicating the number of blocked items; you may need to click the button to see what's going on and test whether you need to make an exception for this site.

Hi jscher2000, Apologies for the delay in trying your solution and replying. You guessed right: it was Content blocking that was causing the issue so I clicked on the shield and dealt with it that way. Many thanks for your help!