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Tried editing the .xul file as DS advd, but to no avail. Tks anycase for sharing.
Did however manage to get the addon to work as a result on Thunderbird 60.8.0.

Just follow 'PART I' of DS's instructions, skip 'PART II' and just install the .xpi file.
Have edited to reflect the way it worked :

1. Go to the Add-on home page link at right // https://codefisher.org/toolbar_button/

2. On the site's toolbar, hover over "Toolbar Buttons" and select "Custom Button Sets".

3. -At top of page select your locale and applications (just thunderbird in my case)
-Select custom set of icons you want to use
-Leave check on "Offer for download instead of installing" only
-Select preferred settings, icon sizes, update stream

4. Click Download and save .xpi file, and install in Thunderbird

5. Add toolbar buttons in Thunderbird as you would normally do.
Note: in my case the buttons selected in 3. showed up in the customize menu,
but without the icons. However once added on the toolbar, the icons showed and
worked as they always did.