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.
Yes it works for the moment and will probably continue to do so for at least another year at the time of writing. But developer edition runs with e10s enabled, so your addon is probably facing e10s compatibility issues which will also hit the release channel sooner than discontinuation of those addon types.
As immediate workaround you can disable it in about:preferences, in the short term you will have to fix those issues, in the mid and long term you'll have to migrate to newer addon frameworks.