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Without modifying the intall.rdf version 1.6.4 of this addon is still not compatible with the latest Thunderbird, despite it being stated in the version info.
Just replace the <em:id> used with the following: "{3550f703-e582-4d05-9a08-453d09bdfdc6}"
and increase the max version to meet your version.
That way it works like a charm.