My device could be one of the kind but I figured this might worth filing. I can |su -c| from adb shell but I cannot |adb root|. It looks like somewhere in the code we try to execute chmod as non-root. Change the adb.py and replace all |self.shell_output("chmod| with |self.shell_output("su -c chmod| fixes this. ==== $ mach mochitest dom/animation/test/mozilla/test_restyles.html 0:01.08 adb INFO su -c supported 0:01.29 adb INFO su 0 supported 0:01.73 adb INFO /system/bin/ls -a supported 0:01.84 adb INFO Native cp support: True 0:01.96 adb INFO Native chmod -R support: True 0:08.83 adb INFO su -c supported 0:09.05 adb INFO su 0 supported 0:09.38 adb INFO /system/bin/ls -a supported 0:09.50 adb INFO Native cp support: True 0:09.61 adb INFO Native chmod -R support: True 0:16.61 INFO Android sdk version '22'; will use this to filter manifests 0:16.61 INFO Checking for ssltunnel processes... 0:16.62 INFO Checking for xpcshell processes... 0:16.64 SUITE_START: mochitest-plain - running 1 tests 0:16.64 INFO Running manifest: dom/animation/test/mochitest.ini 0:16.64 INFO The following extra prefs will be set: dom.animations-api.core.enabled=true 0:17.90 ERROR Automation Error: Unable to copy test modules to device. 0:17.91 ERROR Automation Error: Exception caught while running tests Traceback (most recent call last): File "/Users/timdream/repo/gecko/obj-arm-linux-androideabi/_tests/testing/mochitest/runtestsremote.py", line 353, in run_test_harness retVal = mochitest.runTests(options) File "/Users/timdream/repo/gecko/obj-arm-linux-androideabi/_tests/testing/mochitest/runtests.py", line 2607, in runTests res = self.runMochitests(options, tests_in_manifest) File "/Users/timdream/repo/gecko/obj-arm-linux-androideabi/_tests/testing/mochitest/runtests.py", line 2402, in runMochitests result = self.doTests(options, testsToRun) File "/Users/timdream/repo/gecko/obj-arm-linux-androideabi/_tests/testing/mochitest/runtests.py", line 2697, in doTests self.manifest = self.buildProfile(options) File "/Users/timdream/repo/gecko/obj-arm-linux-androideabi/_tests/testing/mochitest/runtestsremote.py", line 219, in buildProfile self.device.chmod(self.remoteModulesDir, recursive=True) File "/Users/timdream/repo/gecko/central/testing/mozbase/mozdevice/mozdevice/adb.py", line 1457, in chmod timeout=timeout, root=root) File "/Users/timdream/repo/gecko/central/testing/mozbase/mozdevice/mozdevice/adb.py", line 1150, in shell_output raise ADBProcessError(adb_process) ADBProcessError: args: adb -s 04ea83110e984e0a wait-for-device shell chmod -R 777 /storage/sdcard0/tests/modules; echo rc=$?, exitcode: 10, stdout: Unable to chmod /storage/sdcard0/tests/modules: Operation not permitted 0:18.45 INFO Buffered messages finished 0:18.45 SUITE_END 0:18.45 Overall Summary =============== mochitest-plain ~~~~~~~~~~~~~~~ Ran 0 checks () Expected results: 0 OK

This is a Nexus 4 with latest official ROM.

Cool it's fixed already at https://searchfox.org/mozilla-central/rev/9f3da81290054c5b8955bb67ff98cae66676f745/testing/mochitest/runtestsremote.py#219