Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

A couple thoughts in advance:

Some users have reported that setting Firefox to use automatic private browsing was incompatible with Outlook sites. For example, logging in doesn't work.

If you have the problem where you want to open a message in its own window, but the window is blank or says you can close the window now, that can be caused by blocking Outlook sites from setting persistent cookies. You might need to create an exception. More info in the following thread: https://support.mozilla.org/questions/1287025