diff --git a/testing/mochitest/mochitest_options.py b/testing/mochitest/mochitest_options.py
index ef21924761fb..40fcd2da6f92 100644
--- a/testing/mochitest/mochitest_options.py
+++ b/testing/mochitest/mochitest_options.py
@@ -406,6 +406,11 @@ class MochitestArguments(ArgumentContainer):
           "dest": "e10s",
           "help": "Run tests with electrolysis preferences and test filtering disabled.",
           }],
+        [["--enable-fission"],
+         {"action": "store_true",
+          "default": False,
+          "help": "Run tests with fission (site isolation) enabled.",
+          }],
         [["--store-chrome-manifest"],
          {"action": "store",
           "help": "Destination path to write a copy of any chrome manifest "
@@ -824,6 +829,9 @@ class MochitestArguments(ArgumentContainer):
             parser.error("mochitest-{} does not support e10s, try again with "
                          "--disable-e10s.".format(options.flavor))
 
+        if options.enable_fission:
+            options.extraPrefs.append("fission.autostart=1")
+
         options.leakThresholds = {
             "default": options.defaultLeakThreshold,
             "tab": options.defaultLeakThreshold,
diff --git a/testing/mochitest/runtests.py b/testing/mochitest/runtests.py
index 00f86ffee79f..67c54c076f7a 100644
--- a/testing/mochitest/runtests.py
+++ b/testing/mochitest/runtests.py
@@ -2614,6 +2614,13 @@ toolbar#nav-bar {
 
         tests = self.getActiveTests(options)
         self.logPreamble(tests)
+
+        if mozinfo.info['fission'] and not mozinfo.info['e10s']:
+            # Make sure this is logged *after* suite_start so it gets associated with the
+            # current suite in the summary formatters.
+            self.log.error("Fission is not supported without e10s.")
+            return 1
+
         tests = [t for t in tests if 'disabled' not in t]
 
         # Until we have all green, this does not run on a11y (for perf reasons)