Legacy Thunderbird addon in TB63: Services is not defined

I’m attempting to update a non-bootstrap legacy addon (not one I wrote) to Thunderbird 63. (I already got it working in 60.) I’m following this:

I have replaced references to stringbundle with lines like:
var bundle = Services.strings.createBundle(…)

I get this console message (though not on installation - on restart):
ReferenceError: Services is not defined

I have tried adding this to the js files:
but no effect.

What do I have to do?

Whatever this was, the error disappeared which I switched to 64b4