--- a/testing/mochitest/mochitest_options.py
+++ b/testing/mochitest/mochitest_options.py
@@ -784,22 +784,16 @@ class MochitestArguments(ArgumentContain
                 if not os.path.isfile(f):
                     parser.error(
                         'Missing binary %s required for '
                         '--use-test-media-devices' % f)
 
         if options.nested_oop:
             options.e10s = True
 
-        # a11y and chrome tests don't run with e10s enabled in CI
-        if options.a11y or options.chrome:
-            options.e10s = False
-
-        mozinfo.update({"e10s": options.e10s})  # for test manifest parsing.
-
         options.leakThresholds = {
             "default": options.defaultLeakThreshold,
             "tab": 10000,  # See dependencies of bug 1051230.
             # GMP rarely gets a log, but when it does, it leaks a little.
             "geckomediaplugin": 20000,
         }
 
         # XXX We can't normalize test_paths in the non build_obj case here,

--- a/testing/mochitest/runtests.py
+++ b/testing/mochitest/runtests.py
@@ -2185,16 +2185,22 @@ class MochitestDesktop(MochitestBase):
         if options.bisectChunk and options.bisectChunk in self.result:
             bisect.print_summary()
 
         return result
 
     def runTests(self, options):
         """ Prepare, configure, run tests and cleanup """
 
+        # a11y and chrome tests don't run with e10s enabled in CI. Need to set
+        # this here since |mach mochitest| sets the flavor after argument parsing.
+        if options.a11y or options.chrome:
+            options.e10s = False
+        mozinfo.update({"e10s": options.e10s})  # for test manifest parsing.
+
         self.setTestRoot(options)
 
         # Despite our efforts to clean up servers started by this script, in practice
         # we still see infrequent cases where a process is orphaned and interferes
         # with future tests, typically because the old server is keeping the port in use.
         # Try to avoid those failures by checking for and killing orphan servers before
         # trying to start new ones.
         self.killNamedOrphans('ssltunnel')