--- a/mozilla-tests/config.py
+++ b/mozilla-tests/config.py
@@ -1557,17 +1557,17 @@ PLATFORM_UNITTEST_VARS = {
                     'config_files': ["web_platform_tests/prod_config.py"],
                 },
                 'mozbase': {
                     'config_files': ["unittests/mac_unittest.py"],
                 },
             },
         },
         'yosemite': {
-            'opt_unittest_suites': [],
+            'opt_unittest_suites': UNITTEST_SUITES['opt_unittest_suites'][:],
             'debug_unittest_suites': [],
             'suite_config': {
                 'mochitest': {
                     'config_files': ["unittests/mac_unittest.py"],
                 },
                 'mochitest-e10s': {
                     'config_files': ["unittests/mac_unittest.py"],
                 },
@@ -1926,19 +1926,16 @@ for platform in BRANCHES['holly']['platf
         slave_p['opt_unittest_suites'] = MOCHITEST + REFTEST_ONE_CHUNK + REFTEST_NO_IPC + MOCHITEST_DT
         slave_p['debug_unittest_suites'] = MOCHITEST + REFTEST_ONE_CHUNK + REFTEST_NO_IPC + MOCHITEST_DT_3
 
         # Enable content sandbox tests for Windows bit
         if slave_platform in PLATFORMS['win64']['slave_platforms'] or slave_platform in PLATFORMS['win32']['slave_platforms']:
             slave_p['opt_unittest_suites'] += MOCHITEST_CSB
             slave_p['debug_unittest_suites'] += MOCHITEST_CSB
 
-# Enable Yosemite testing on select branches only
-delete_slave_platform(BRANCHES, PLATFORMS, {'macosx64': 'yosemite'}, branch_exclusions=['try'])
-
 # Run Jetpack tests everywhere except on versioned B2G branches.
 for name in [x for x in BRANCHES.keys() if not x.startswith('mozilla-b2g')]:
     branch = BRANCHES[name]
     for pf in PLATFORMS:
         if pf not in branch['platforms']:
             continue
         # Skip these platforms
         if pf in ('linux64-asan', ):
@@ -2191,16 +2188,35 @@ for branch in BRANCHES.keys():
         enabled_platforms_for_os = BRANCHES[branch]['platforms'][os]['talos_slave_platforms']
         for s in SUITES.iterkeys():
             tests_key = '%s_tests' % s
             if tests_key in BRANCHES[branch]:
                 tests = list(BRANCHES[branch]['%s_tests' % s])
                 tests[3] = [x for x in tests[3] if x not in platforms_for_os or x in enabled_platforms_for_os]
                 BRANCHES[branch]['%s_tests' % s] = tuple(tests)
 
+# bug 1126493 Enable Yosemite testing on select branches only
+# keep debug tests on 10.8 until the source of the slowness is found in bug 1125998
+include_yosemite = ['try']
+for platform in PLATFORMS.keys():
+    # See Bug 997946 - skip these on OS X 10.8 due to limited capacity
+    for name, branch in items_at_least(BRANCHES, 'gecko_version', 39):
+        if platform not in branch['platforms']:
+            continue
+        for slave_platform in branch['platforms'][platform]:
+            if slave_platform not in ['mountainlion', 'yosemite']:
+                continue
+            if name not in include_yosemite:
+                include_yosemite.append(name)
+if len(include_yosemite) > 0:
+    delete_slave_platform(BRANCHES, PLATFORMS, {'macosx64': 'yosemite'}, branch_exclusions=include_yosemite)
+    for branch in include_yosemite:
+        BRANCHES[branch]['platforms']['macosx64']['mountainlion']['opt_unittest_suites'] = []
+        #todo disable talos too
+
 # Versioned b2g branches shouldn't run mochitest-browser-chrome on linux debug builds
 for name in [x for x in BRANCHES.keys() if x.startswith('mozilla-b2g')]:
     branch = BRANCHES[name]
     if 'linux' in branch['platforms'] and 'ubuntu32_vm' in branch['platforms']['linux']:
         for chunked_bc in MOCHITEST_BC_3:
             branch['platforms']['linux']['ubuntu32_vm']['debug_unittest_suites'].remove(chunked_bc)
     if 'linux64' in branch['platforms'] and 'ubuntu64_vm' in branch['platforms']['linux64']:
         for chunked_bc in MOCHITEST_BC_3: