If Firefox shows MSN every time you launch a new window (Ctrl+N), you probably need to change your home page. More info in the following article: [[How to set the home page]]. Could you describe the sequence of steps that leads up to Microsoft Edge opening? Is that when you load certain kinds of content in Firefox, or when you take actions external to Firefox? If the problem is with links in the MS Office Outlook 365 app, see the last section of the following article: [[Setting Firefox as the default browser does not work - What to do]].