Since Thunderbird 38 release the custom icons in the folder tree are not
overwritten.

Apparently Thunderbird has removed Dict.jsm while moving forward into
JavaScript ES6. This means the code that loads and applies the icons in the
folder tree has to be forked so it supports the old platform (Dictionary object
for Postbox) and the modern one (Map object for Thunderbird & SeaMonkey)

Created an attachment (id=8065) [details]
4.1 prerelease 331

THIS. Should fix it :-) Works in Thunderbird 38.0.1 release

Needs to be regression tested in Postbox