Now, the screenshot doesn't have a URL in the address bar, suggesting it might be a built-in special page. But it also has a generic "Page title" in the tab title, so it looks like a modified stock image rather than an actual UI screenshot.
The screenshot on Troy Hunt's blog does show "https://www.mozilla.org/firefoxmonitor", but it's otherwise the same screenshot with the same generic "Page title", so it's probably equally fake.
(I tried to look for the actual bugzilla bug for this for more info, but didn't find anything.)
Firefox Monitor is just a page on the official Mozilla website.