I have a strange behavior in ArcGIS Online when using Firefox. I get the text "No Legend" on the Content Tab under my layers in the map service if I do the following:
1) Expand the layer so I can see the legend, before the layer turned on and then
2) Enable popups.
For the rest of the live of the map the layer will not show its legend in the contents tab (ie just text "No Legend" will appear), however it will show it in the legend tab.
The work around is:
1) Don't expand the layer first
2) Turn the layer on
3) Enable the popup
The other work around if you screwed up is:
1) Disable the popup
I then can see the lend under the layer in the contents tab
In the above case I am using a secured layer from ArcServer 10.3.1 over https. This layer has first been created using the equivelent of a layer file in AGO, by first