Some Firefox users have reported (and ITS has confirmed) that word wrap in OWA is not working correctly when typing a message (the typed text does not wrap to a new line). Microsoft acknowledges the problem and is investigating. The workaround is to use a different Web browser, such as Chrome or Internet Explorer (Windows), or Chrome or Safari (Mac).
https://www.ithaca.edu/intercom/article.php/20140428124120880