--- a/mail/test/mozmill/runtestlist.py
+++ b/mail/test/mozmill/runtestlist.py
@@ -1,17 +1,17 @@
 #!/usr/bin/env python
 # 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 glob
 import optparse
 import sys
 import os
-import shutil
 import subprocess
 import logging
 
 SCRIPT_DIRECTORY = os.path.abspath(os.path.realpath(os.path.dirname(sys.argv[0])))
 
 class RunTestListOptions(optparse.OptionParser):
     """Parsed run test list command line options."""
     def __init__(self, **kwargs):
@@ -74,25 +74,41 @@ options, args = parser.parse_args()
 if options.binary == "" or options.list == "":
     parser.print_help()
     sys.exit(1)
 
 totalTestErrors = 0
 totalTestPasses = 0
 totalDirectories = 0
 
-tests = open(options.list, "rt").readlines()
+tests = [t.strip() for t in open(options.list, "rt").readlines()]
 
 if options.total_chunks > 1:
-    tests_per_chunk = float(len(tests)) / options.total_chunks
+    test_counts = {}
+    total_test_count = 0
+    for t in tests:
+        if os.path.isdir(os.path.join(SCRIPT_DIRECTORY, t)):
+            test_counts[t] = len(glob.glob(os.path.join(SCRIPT_DIRECTORY, t, "test*.js")))
+        else:
+            test_counts[t] = 1
+        total_test_count += test_counts[t]
+
+    tests_per_chunk = float(total_test_count) / options.total_chunks
     start = int(round((options.this_chunk - 1) * tests_per_chunk))
     end = int(round(options.this_chunk * tests_per_chunk))
 
-    tests = (t for t in tests[start:end])
-
+    chunk_tests = []
+    cumulative_test_count = 0
+    for t in tests:
+        if cumulative_test_count >= end:
+            break
+        if cumulative_test_count >= start:
+            chunk_tests.append(t)
+        cumulative_test_count += test_counts[t]
+    tests = chunk_tests
 
 for directory in tests:
     log.info("INFO | (runtestlist.py) | Running directory: %s",
              directory.rstrip())
     if options.dir != "":
         testDirectory = os.path.join(options.dir, directory.rstrip())
     else:
         testDirectory = directory.rstrip()

--- a/taskcluster/ci/test/tests.yml
+++ b/taskcluster/ci/test/tests.yml
@@ -38,17 +38,16 @@ xpcshell:
     treeherder-symbol: X(X)
     run-on-projects: built-projects
     docker-image:
         in-tree: "desktop1604-test"
     chunks:
         by-test-platform:
             linux32/debug: 8
             linux64/debug: 8
-            macosx64.*: 2
             default: 4
     instance-size: default
     max-run-time: 5400
     e10s: false
     allow-software-gl-layers: false
     mozharness:
         script: desktop_unittest.py
         config: