--- a/testing/marionette/session.js
+++ b/testing/marionette/session.js
@@ -170,17 +170,17 @@ session.Proxy = class {
           if (Number.isInteger(this.socksProxyPort)) {
             Preferences.set("network.proxy.socks_port", this.socksProxyPort);
           }
           if (this.socksVersion) {
             Preferences.set("network.proxy.socks_version", this.socksVersion);
           }
         }
 
-        if (this.noProxy && this.noProxy.length > 0) {
+        if (this.noProxy) {
           Preferences.set("network.proxy.no_proxies_on", this.noProxy.join(", "));
         }
         return true;
 
       case "pac":
         Preferences.set("network.proxy.type", 2);
         Preferences.set(
             "network.proxy.autoconfig_url", this.proxyAutoconfigUrl);

--- a/testing/marionette/test_session.js
+++ b/testing/marionette/test_session.js
@@ -162,16 +162,23 @@ add_test(function test_Proxy_init() {
     equal(Preferences.get("network.proxy.no_proxies_on"), "foo, bar");
     equal(Preferences.get(`network.proxy.${proxy}`), "foo");
     equal(Preferences.get(`network.proxy.${proxy}_port`), 42);
     if (proxy === "socks") {
       equal(Preferences.get(`network.proxy.${proxy}_version`), 4);
     }
   }
 
+  // empty no proxy should reset default exclustions
+  p = new session.Proxy();
+  p.proxyType = "manual";
+  p.noProxy = [];
+  ok(p.init());
+  equal(Preferences.get("network.proxy.no_proxies_on"), "");
+
   run_next_test();
 });
 
 add_test(function test_Proxy_toString() {
   equal(new session.Proxy().toString(), "[object session.Proxy]");
 
   run_next_test();
 });