If it's working in Chrome then there probably isn't anything wrong with the setup itself. Things to try:
- Update to the latest Firefox browser if you have not already done this.
- Check to see if the issue persists if you clear the cache.
- Check to see if the issue occurs if you use an in-private window.
- In Firefox go to Settings > privacy > remove individual cookies
Search for Outlook and delete everything. Then search for office delete again. Or, if you don't care about logins for everything, remove all cookies. (This isn't a convenient solution but may be necessary if there's a caching problem.)