Which Zend Server version did you upgrade to 9.1?
[Update] F5 should solve this (below is the rest of the steps)
Have you tried to clear cache, use private window, and close Firefox (not in task manager) and open fresh?
This can also be related to a browser extension or plugin, if you can relate this to a newly installed / updated extension or user script / style etc.
If it is still the case after the above checks, please share a screenshot for review.
Note: Firefox can be updated to 52.0.2, we will not be testing a specific (older) version but the latest.
Zend Support Team