I'm not a TB user, but it does seem that Moz has made it exceedingly difficult to add any search engine to the program. You may want to try using the SearchWith extension: http://searchwith.mozdev.org/
posted by x.15a2 Community Leader2 years and 2 months ago Link

Thanks for the info. I did manage to add DDG as the search engine for Thunderbird in a roundabout way.
posted by ineuw 2 years and 2 months ago Link