Hi, I haven't seen that, but I'm not sure whether I'm using the same layout you are.

As a starting point, do you mean the "consumer" version of Outlook online on the Live domain --

https://outlook.live.com/mail/

-- or the desktop application, or the online version on the Office / Microsoft 365 site?

I don't know what version. I used to use Hotmail and then I had to click on the New Outlook button.  If I type in https://outlook.live.com/mail/ from my Firefox browser, nothing comes up but a blank page.  I had to select the new outlook app from the start screen and pin the app to the task bar.  When I click on the app from the task bar the new outlook comes up but there is no address bar at all to indicate the path or URL.

Maybe the one pinned to your Taskbar is using Edge as the browser engine instead of Firefox? It can be hard to tell when there are no toolbars to give it away. Perhaps clicking a link in a message will reveal which browser that is?

But either way, when you click the outlook.live.com link in Firefox, if you aren't already signed in, the site should redirect you to the login page. Maybe the redirect is blocked or broken? You could try a direct link:

https://login.live.com/login.srf

That doesn't tell the site where to send you next, though, so you need to use your email link again after that.