--- a/taskcluster/ci/desktop-test/tests.yml
+++ b/taskcluster/ci/desktop-test/tests.yml
@@ -144,16 +144,18 @@ mochitest:
         script: mozharness/scripts/desktop_unittest.py
         no-read-buildbot-config: true
         chunked: true
         config:
             - mozharness/configs/unittests/linux_unittest.py
             - mozharness/configs/remove_executables.py
         extra-options:
             - --mochitest-suite=plain-chunked
+    # Bug 1281241: migrating to m3.large instances
+    instance-size: legacy
 
 mochitest-a11y:
     description: "Mochitest a11y run"
     suite: mochitest/a11y
     treeherder-symbol: tc-M(a11y)
     loopback-video: true
     e10s: false
     mozharness:
@@ -179,16 +181,18 @@ mochitest-browser-chrome:
     mozharness:
         script: mozharness/scripts/desktop_unittest.py
         no-read-buildbot-config: true
         config:
             - mozharness/configs/unittests/linux_unittest.py
             - mozharness/configs/remove_executables.py
         extra-options:
             - --mochitest-suite=browser-chrome-chunked
+    # Bug 1281241: migrating to m3.large instances
+    instance-size: legacy
 
 mochitest-chrome:
     description: "Mochitest chrome run"
     suite: mochitest/chrome
     treeherder-symbol: tc-M(c)
     loopback-video: true
     chunks: 3
     e10s: false

--- a/taskcluster/taskgraph/transforms/tests/make_task_description.py
+++ b/taskcluster/taskgraph/transforms/tests/make_task_description.py
@@ -120,19 +120,20 @@ def docker_worker_setup(config, test, ta
 
     installer_url = ARTIFACT_URL.format('<build>', mozharness['build-artifact-name'])
     test_packages_url = ARTIFACT_URL.format('<build>',
                                             'public/build/target.test_packages.json')
     mozharness_url = ARTIFACT_URL.format('<build>',
                                          'public/build/mozharness.zip')
 
     taskdesc['worker-type'] = {
-        'default': 'aws-provisioner-v1/desktop-test',
+        'default': 'aws-provisioner-v1/desktop-test-large',
         'large': 'aws-provisioner-v1/desktop-test-large',
         'xlarge': 'aws-provisioner-v1/desktop-test-xlarge',
+        'legacy': 'aws-provisioner-v1/desktop-test',
     }[test['instance-size']]
 
     worker = taskdesc['worker'] = {}
     worker['implementation'] = test['worker-implementation']
 
     docker_image = test.get('docker-image')
     assert docker_image, "no docker image defined for a docker-worker/docker-engine task"
     if isinstance(docker_image, dict):

--- a/taskcluster/taskgraph/transforms/tests/test_description.py
+++ b/taskcluster/taskgraph/transforms/tests/test_description.py
@@ -72,18 +72,18 @@ test_description_schema = Schema({
     # and treeherder group.
     Required('e10s', default='both'): Any(
         bool, 'both',
         {'by-test-platform': {basestring: Any(bool, 'both')}},
     ),
 
     # The EC2 instance size to run these tests on.
     Required('instance-size', default='default'): Any(
-        Any('default', 'large', 'xlarge'),
-        {'by-test-platform': {basestring: Any('default', 'large', 'xlarge')}},
+        Any('default', 'large', 'xlarge', 'legacy'),
+        {'by-test-platform': {basestring: Any('default', 'large', 'xlarge', 'legacy')}},
     ),
 
     # Whether the task requires loopback audio or video (whatever that may mean
     # on the platform)
     Required('loopback-audio', default=False): bool,
     Required('loopback-video', default=False): bool,
 
     # The worker implementation for this test, as dictated by policy and by the