Some Mac users are unable to search in OWA using Safari or Firefox. This is a known issue related to ongoing system updates in Office 365.
As a workaround, consider using Google Chrome as an alternative web browser.
Select the Chrome menu on the browser toolbar and select About Google Chrome. The current version number is the series of numbers beneath the "Google Chrome" heading. Chrome will check for updates when you are on this page. Select Relaunch to apply any available update.