Fixed it. Had to delete and reinstall Firefox, log in again, and re-sync.

Same problem, but for iOS 11.3 on iPhone 7plus

Did you delete it on the phone or desktop? or both?

I deleted it from the iPhone entirely, probably powered the phone off and on, and then reinstalled. If i found where to delete firefox cached data from the iPhone, i would have done that too. I did not delete desktop profiles, but that probably would have helped in my situation. The iPhone would sync again, but many of bookmarks are in very bad shape. Most of them appear to be missing even on the desktop. FirefoxESR 60 just came out yesterday so that will help me as CentOS uses that by default. FirefoxESR 60 will now have the Quantum benefits and the many changes Quantum brings to bookmarks. Running different versions of Firefox that sync to the same account caused problems.