IDM has consistently refused to work on a addon for Palemoon. Until now!! 
Problem is that the usual xpi files in your IDM folder still won't work.
Frustrated with this I found was is needed on Palemoons forum.
xpi file:
Site: https://www.upload.ee
Sharecode: /files/10180053/idmmzcc-palemoon.rar.html
To use: Click "Open file..." in the "file" tab of your browser and select the xpi file.
Source:
https://forum.palemoon.org/viewtopic.php?f=47&t=22042&p=169826&hilit=IDM#p170010