Legacy overlay addon migration - missing groupbox labels in prefs window

I migrated an addon (not one I wrote) to TB 64+. It works, but the XUL preferences window is missing its groupbox labels.

The code is here:

and the issue here:

Can anybody here shed any light on this?
Dave (aka revad)

I asked in bugzilla and got confirmation that, as I suspected, this is caused by the removal of groupbox bindings under bug 1493844: https://bugzilla.mozilla.org/show_bug.cgi?id=1476259#c17