I remember it has been mentioned somewhere that Mozilla disabled execution of any extensions on addons.mozilla.org for security reasons. In about:config I found a preference extensions.webextensions.restrictedDomains where this domain is listed, but clearing the value doesn't have any effect on addons.mozilla.org. I'm using Iceweasel 1:61.0.2-1.parabola2 in Parabola GNU/Linux-libre distro.

Most likely this is not a bug in LibreJS and perhaps there's no way to overcome it for individual extension. But I'm logging this issue anyway, to raise the awareness. Perhaps it would be wise to mention this issue in the addon description, or forward it to maintainers of Iceweasel and IceCat so that they can patch the code to make an exception for LibreJS to work on addons.mozilla.org