--- a/taskcluster/docker/image_builder/download-and-compress
+++ b/taskcluster/docker/image_builder/download-and-compress
@@ -1,9 +1,9 @@
-#!/usr/bin/python2.7
+#!/usr/bin/python3 -u
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 import os
 import sys
 import time
 

--- a/taskcluster/docker/image_builder/setup.sh
+++ b/taskcluster/docker/image_builder/setup.sh
@@ -4,25 +4,26 @@ set -v -e -x
 export DEBIAN_FRONTEND=noninteractive
 
 # Update apt-get lists
 apt-get update -y
 
 # Install dependencies
 apt-get install -y --no-install-recommends \
     socat \
+    python-requests \
+    python-requests-unixsocket \
     python3.5 \
     python3-minimal \
-    python \
-    python-requests \
-    python-requests-unixsocket
+    python3-requests \
+    python3-requests-unixsocket
 
 # Extra dependencies only needed for image building. Will be removed at
 # end of script.
-apt-get install -y python-pip
+apt-get install -y python-pip python3-pip
 
 # Install mercurial
 # shellcheck disable=SC1091
 . /setup/common.sh
 # shellcheck disable=SC1091
 . /setup/install-mercurial.sh
 
 # Install build-image.sh script
@@ -44,21 +45,24 @@ tooltool_fetch <<EOF
     "digest": "72b1fc542e5af36fc660d7b8d3882f0a25644d3b66316293717aabf9ba8cf578e49e2cf45e63e962c5535ec1f8b3e83248c379d34b0cab2ef1a950205ad153ce",
     "algorithm": "sha512",
     "filename": "zstandard-0.9.0.tar.gz"
   }
 ]
 EOF
 )
 
+# We need to install for both Python 2 and 3 because `mach taskcluster-load-image`
+# uses Python 2 and `download-and-compress` uses Python 3.
 /usr/bin/pip -v install /setup/zstandard-0.9.0.tar.gz
+/usr/bin/pip3 -v install /setup/zstandard-0.9.0.tar.gz
 
 # python-pip only needed to install python-zstandard. Removing it removes
 # several hundred MB of dependencies from the image.
-apt-get purge -y python-pip
+apt-get purge -y python-pip python3-pip
 
 # Purge apt-get caches to minimize image size
 apt-get auto-remove -y
 apt-get clean -y
 rm -rf /var/lib/apt/lists/
 
 # Remove this script
 rm -rf /setup/