Right now ArcGIS Online has support for Firlefly points. But there is no equivalent for lines and polygons supported by Firefly style. It would be great to have an option to create web maps with Firefly lines and polygons without the need for publishing WebMap Service in to ArcGIS Server. Firefly style is working with vector tiles fine, so I guess it should be also technically possible to do the same with JavaScript API and ArcGIS Online.