Office 365 – Firefox
Microsoft have received reports of an issue with some customers using Firefox that means the toolbar above e-mails disappear when composing a message.
Microsoft engineers are fixing this at the moment, but the current workaround for this is to put the browser into full screen mode, where the tool bar should re-appear.
To do this, press F11 on your keyboard and the browser will expand to full screen mode. Whilst in full screen mode the address bar and other icons will minimize to the top of the screen. To make this show again, hold your mouse at the top of the browser and it will maximize again. To exit full screen, press F11 again.
Alternatively, you can use a different browser such a Google Chrome or a later version of Internet Explorer (9+).