/usr/share/icedove/extensions/ is in icedove-bidiui 0.9.7-1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/icedove/extensions/{54e72d7b-ed31-4736-854f-f517358f21ee}/
/usr/share/icedove/extensions/{54e72d7b-ed31-4736-854f-f517358f21ee}/chrome/
/usr/share/icedove/extensions/{54e72d7b-ed31-4736-854f-f517358f21ee}/chrome.manifest
/usr/share/icedove/extensions/{54e72d7b-ed31-4736-854f-f517358f21ee}/chrome/bidimailpack.jar
/usr/share/icedove/extensions/{54e72d7b-ed31-4736-854f-f517358f21ee}/defaults/
/usr/share/icedove/extensions/{54e72d7b-ed31-4736-854f-f517358f21ee}/defaults/preferences/
/usr/share/icedove/extensions/{54e72d7b-ed31-4736-854f-f517358f21ee}/defaults/preferences/bidimailui.js
/usr/share/icedove/extensions/{54e72d7b-ed31-4736-854f-f517358f21ee}/install.rdf
/usr/share/icedove/extensions/{54e72d7b-ed31-4736-854f-f517358f21ee}/platform/
/usr/share/icedove/extensions/{54e72d7b-ed31-4736-854f-f517358f21ee}/platform/Darwin/
/usr/share/icedove/extensions/{54e72d7b-ed31-4736-854f-f517358f21ee}/platform/Darwin/chrome/
/usr/share/icedove/extensions/{54e72d7b-ed31-4736-854f-f517358f21ee}/platform/Darwin/chrome.manifest
/usr/share/icedove/extensions/{54e72d7b-ed31-4736-854f-f517358f21ee}/platform/Darwin/chrome/bidimailpack-mac-skin.jar