It is mentioned that Legacy add-ons are deprecated(here). Though I am trying to make a very minor update to an existing add-on which is working fine on release channel.
I installed the add-on in the FF Developer Edition and found some issues:
document.commandDispatcher.focusedWindow.getSelection().toString()
(to get the selected text) is not returning the selected value with no error.The overlay window height and width is set using preferences but it seems to be not reading.