diff --git a/netwerk/protocol/http/UserAgentOverrides.jsm b/netwerk/protocol/http/UserAgentOverrides.jsm
index e4186782acf8..c5cbd0d67744 100644
--- a/netwerk/protocol/http/UserAgentOverrides.jsm
+++ b/netwerk/protocol/http/UserAgentOverrides.jsm
@@ -32,6 +32,7 @@ var gInitialized = false;
 var gOverrideFunctions = [
   function (aHttpChannel) UserAgentOverrides.getOverrideForURI(aHttpChannel.URI)
 ];
+var gBuiltUAs = new Map;
 
 this.UserAgentOverrides = {
   init: function uao_init() {
@@ -53,6 +54,9 @@ this.UserAgentOverrides = {
     UserAgentUpdates.init(function(overrides) {
       gOverrideForHostCache.clear();
       if (overrides) {
+        for (let domain in overrides) {
+          overrides[domain] = getUserAgentFromOverride(overrides[domain]);
+        }
         overrides.get = function(key) this[key];
       }
       gUpdatedOverrides = overrides;
@@ -129,6 +133,22 @@ this.UserAgentOverrides = {
   }
 };
 
+function getUserAgentFromOverride(override)
+{
+  let userAgent = gBuiltUAs.get(override);
+  if (userAgent !== undefined) {
+    return userAgent;
+  }
+  let [search, replace] = override.split("#", 2);
+  if (search && replace) {
+    userAgent = DEFAULT_UA.replace(new RegExp(search, "g"), replace);
+  } else {
+    userAgent = override;
+  }
+  gBuiltUAs.set(override, userAgent);
+  return userAgent;
+}
+
 function buildOverrides() {
   gOverrides.clear();
   gOverrideForHostCache.clear();
@@ -141,17 +161,7 @@ function buildOverrides() {
 
   for (let domain of domains) {
     let override = gPrefBranch.getCharPref(domain);
-    let userAgent = builtUAs.get(override);
-
-    if (userAgent === undefined) {
-      let [search, replace] = override.split("#", 2);
-      if (search && replace) {
-        userAgent = DEFAULT_UA.replace(new RegExp(search, "g"), replace);
-      } else {
-        userAgent = override;
-      }
-      builtUAs.set(override, userAgent);
-    }
+    let userAgent = getUserAgentFromOverride(override);
 
     if (userAgent != DEFAULT_UA) {
       gOverrides.set(domain, userAgent);
diff --git a/netwerk/test/mochitests/test_user_agent_updates.html b/netwerk/test/mochitests/test_user_agent_updates.html
index 657032d402cf..077e7e8fa85e 100644
--- a/netwerk/test/mochitests/test_user_agent_updates.html
+++ b/netwerk/test/mochitests/test_user_agent_updates.html
@@ -30,6 +30,12 @@ const DEFAULT_UA = navigator.userAgent;
 const UA_OVERRIDE = "DummyUserAgent";
 const UA_ALT_OVERRIDE = "AltUserAgent";
 
+const UA_PARTIAL_FROM = "\\wozilla"; // /\wozilla
+const UA_PARTIAL_SEP = "#";
+const UA_PARTIAL_TO = UA_OVERRIDE;
+const UA_PARTIAL_OVERRIDE = UA_PARTIAL_FROM + UA_PARTIAL_SEP + UA_PARTIAL_TO;
+const UA_PARTIAL_EXPECTED = DEFAULT_UA.replace(new RegExp(UA_PARTIAL_FROM, 'g'), UA_PARTIAL_TO);
+
 function getUA(host) {
   var url = location.pathname;
   url = host + url.slice(0, url.lastIndexOf('/')) + '/user_agent.sjs';
@@ -78,6 +84,12 @@ const OVERRIDES = [
     expected: SpecialPowers.Services.appinfo.OS,
     host: 'http://sub1.test2.example.org'
   },
+  {
+    domain: 'sub2.test2.example.org',
+    override: UA_PARTIAL_OVERRIDE,
+    expected: UA_PARTIAL_EXPECTED,
+    host: 'http://sub2.test2.example.org'
+  },
 ];
 
 function getServerURL() {