Hello, Using a Macbook Air and Firefox. Just last week, my OWA new emails and replies to emails suddenly began opening as a new window - more like a pop-up. I can't find any place to change it back. It used to open within the browser, which is what I prefer. Can anyone help me change it back? Thanks!!

Click the Cog in the Upper Right hand corner and choose options.  Under there click settings.  The first tab should then be Mail.  Look at the Right Pane and you should see something that says reading pane.  That's where you can change that option. 

Hi,

Please switch to another PC and other browser to check this issue. If there is no problem with other PC or browser, then this maybe Firefox settting problem. And please connect Apple support for help.

Best Regards.