Can't install some extensions.

According to this, extensions without an ID in their manifest do not work with IceCat.

Extracting the .xpi file, editing the manifest.json file by adding this block:

    "applications": {
        "gecko": {
            "id": "address@hidden",
            "strict_min_version": "56.0a1"
        }
    }

and rebuilding (Compress to a zip file) the archive solves the issue.

Why IceCat behave this way is still a mystery.