New
#1
From what I can discern from the image is the browser is in the background and the message is in the foreground, appears to be normal. I also use a Webmail link in Firefox and simply double-clicking a message in the list on the Web page opens that message for better reading.
The section on the right with the word "Year" in it is what I should see when I open an email from my inbox.The part that I surrounded in a red box is what opens over that.
The underneath one looks like a Preview pane. The one you outline a message pane.
In most Email applications and Webmail that preview is an option you can turn off.
I do, and just have a message list, click to read message right click to mark as spam.
https://www.bt.com/help/email
It could also be a Browser window/tab setting.
The reason for not having a Preview Pane is because spammers take note of opened Emails and then add you to a mugs list of 'live Email addresses'.
Spammers will also scan sites like this to farm Email addresses, so you should never display it in an image or text form.