--- a/testing/docker/centos6-build/system-setup.sh
+++ b/testing/docker/centos6-build/system-setup.sh
@@ -416,21 +416,51 @@ cd $BUILD
 pip2.7 install peep
 
 # remaining Python utilities are installed with `peep` from upstream
 # repositories; peep verifies file integrity for us
 cat >requirements.txt <<'EOF'
 # sha256: 90pZQ6kAXB6Je8-H9-ivfgDAb6l3e5rWkfafn6VKh9g
 virtualenv==13.1.2
 
-# sha256: wJnELXTi1SC2HdNyzZlrD6dgXAZheDT9exPHm5qaWzA
-mercurial==3.7.3
+# sha256: Sy4-8Z00-h14HLdCVQagXUtrEXK6tp1up4h0F1_fPaY
+Mercurial==3.8.4
 EOF
 peep install -r requirements.txt
 
+mkdir -p /usr/local/mercurial
+cd /usr/local/mercurial
+
+tooltool_fetch <<'EOF'
+[
+{
+    "size": 11849,
+    "digest": "c88d9b8afd6649bd28bbacfa654ebefec8087a01d1662004aae088d485edeb03a92df1193d1310c0369d7721f475b974fcd4a911428ec65936f7e40cf1609c49",
+    "algorithm": "sha512",
+    "filename": "robustcheckout.py"
+}
+]
+EOF
+
+mkdir -p /etc/mercurial
+cat >/etc/mercurial/hgrc <<EOF
+# By default the progress bar starts after 3s and updates every 0.1s. We
+# change this so it shows and updates every 1.0s.
+[progress]
+delay = 1.0
+refresh = 1.0
+
+cacerts = /etc/pki/tls/certs/ca-bundle.crt
+
+[extensions]
+robustcheckout = /usr/local/mercurial/robustcheckout.py
+EOF
+
+chmod 644 /etc/mercurial/hgrc
+
 # TC-VCS
 npm install -g taskcluster-vcs@2.3.18
 
 # Ninja
 cd $BUILD
 tooltool_fetch <<'EOF'
 [
 {