--- a/testing/mozharness/scripts/mobile_l10n.py
+++ b/testing/mozharness/scripts/mobile_l10n.py
@@ -182,16 +182,30 @@ class MobileSingleLocale(MockMixin, Loca
         repack_env = self.query_env(partial_env=c.get("repack_env"),
                                     replace_dict=replace_dict)
         if c.get('base_en_us_binary_url') and c.get('release_config_file'):
             rc = self.query_release_config()
             repack_env['EN_US_BINARY_URL'] = c['base_en_us_binary_url'] % replace_dict
         if 'MOZ_SIGNING_SERVERS' in os.environ:
             repack_env['MOZ_SIGN_CMD'] = \
                 subprocess.list2cmdline(self.query_moz_sign_cmd(formats=['jar']))
+
+        if self.query_is_nightly() or self.query_is_nightly_promotion():
+            if self.query_is_nightly():
+                # Nightly promotion needs to set update_channel but not do all
+                # the 'IS_NIGHTLY' automation parts, like uploading symbols
+                # (for now).
+                repack_env["IS_NIGHTLY"] = "yes"
+            # In branch_specifics.py we might set update_channel explicitly.
+            if c.get('update_channel'):
+                repack_env["MOZ_UPDATE_CHANNEL"] = c['update_channel']
+            else:  # Let's just give the generic channel based on branch.
+                repack_env["MOZ_UPDATE_CHANNEL"] = \
+                    "nightly-%s" % (c['branch'],)
+
         self.repack_env = repack_env
         return self.repack_env
 
     def query_l10n_env(self):
         return self.query_env()
 
     def query_upload_env(self):
         if self.upload_env: