Browsers still contain bugs in Workflow Designer:
1) SP2016 - Firefox 66.0.3 (64-bit) - Set variable action, equals Value: add a Item Property of type Single line of text and attempt to type after it. This occurs in multiple actions.
2) SPOnline - Firefox 66.0.3 (64-bit) - Set variable action, add item property. Unable to delete item property. Same occurs in the Body of a Send an Email action.
3) SPOnline - Chrome 74.0.3729.108 (64-bit) - Recurring issue where dropdown selections are blank because the selections are displayed to the left of the viewable dropdown window. This happens in various dropdown controls (e.g. Advanced Lookup --> Equals dropdowns).
4) IE has many issues but I don't believe that's intended for use anymore.