--- a/b2g/confvars.sh
+++ b/b2g/confvars.sh
@@ -1,16 +1,18 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 MOZ_APP_BASENAME=B2G
 MOZ_APP_VENDOR=Mozilla
 
 MOZ_APP_VERSION=17.0a1
+
+MOZ_UA_OS_AGNOSTIC=1
 MOZ_APP_UA_NAME=Firefox
 
 MOZ_BRANDING_DIRECTORY=b2g/branding/unofficial
 MOZ_OFFICIAL_BRANDING_DIRECTORY=b2g/branding/official
 # MOZ_APP_DISPLAYNAME is set by branding/configure.sh
 
 MOZ_SAFE_BROWSING=
 MOZ_SERVICES_SYNC=1

--- a/config/autoconf.mk.in
+++ b/config/autoconf.mk.in
@@ -17,16 +17,17 @@ MOZ_APP_BASENAME = @MOZ_APP_BASENAME@
 MOZ_APP_VENDOR = @MOZ_APP_VENDOR@
 MOZ_APP_PROFILE = @MOZ_APP_PROFILE@
 MOZ_APP_ID = @MOZ_APP_ID@
 MAR_CHANNEL_ID = @MAR_CHANNEL_ID@
 ACCEPTED_MAR_CHANNEL_IDS = @ACCEPTED_MAR_CHANNEL_IDS@
 MOZ_PROFILE_MIGRATOR = @MOZ_PROFILE_MIGRATOR@
 MOZ_EXTENSION_MANAGER = @MOZ_EXTENSION_MANAGER@
 MOZ_APP_UA_NAME = @MOZ_APP_UA_NAME@
+MOZ_UA_OS_AGNOSTIC = @MOZ_UA_OS_AGNOSTIC@
 MOZ_APP_VERSION = @MOZ_APP_VERSION@
 MOZ_APP_MAXVERSION = @MOZ_APP_MAXVERSION@
 MOZ_MACBUNDLE_NAME = @MOZ_MACBUNDLE_NAME@
 MOZ_MACBUNDLE_ID = @MOZ_MACBUNDLE_ID@
 MOZ_APP_STATIC_INI = @MOZ_APP_STATIC_INI@
 
 MOZ_PKG_SPECIAL = @MOZ_PKG_SPECIAL@
 

--- a/configure.in
+++ b/configure.in
@@ -8406,16 +8406,20 @@ AC_SUBST(MOZ_PROFILE_MIGRATOR)
 AC_SUBST(MOZ_EXTENSION_MANAGER)
 AC_DEFINE_UNQUOTED(MOZ_APP_UA_NAME, "$MOZ_APP_UA_NAME")
 AC_SUBST(MOZ_APP_UA_NAME)
 AC_DEFINE_UNQUOTED(MOZ_APP_UA_VERSION, "$MOZ_APP_VERSION")
 AC_SUBST(MOZ_APP_VERSION)
 AC_SUBST(MOZ_APP_MAXVERSION)
 AC_DEFINE_UNQUOTED(FIREFOX_VERSION,$FIREFOX_VERSION)
 AC_SUBST(FIREFOX_VERSION)
+AC_SUBST(MOZ_UA_OS_AGNOSTIC)
+if test -n "$MOZ_UA_OS_AGNOSTIC"; then
+  AC_DEFINE(MOZ_UA_OS_AGNOSTIC)
+fi
 
 # We can't use the static application.ini data when building against
 # a libxul SDK.
 if test -n "$LIBXUL_SDK"; then
     MOZ_APP_STATIC_INI=
 fi
 AC_SUBST(MOZ_APP_STATIC_INI)
 

--- a/netwerk/protocol/http/nsHttpHandler.cpp
+++ b/netwerk/protocol/http/nsHttpHandler.cpp
@@ -511,19 +511,17 @@ nsHttpHandler::UserAgent()
 }
 
 void
 nsHttpHandler::BuildUserAgent()
 {
     LOG(("nsHttpHandler::BuildUserAgent\n"));
 
     NS_ASSERTION(!mLegacyAppName.IsEmpty() &&
-                 !mLegacyAppVersion.IsEmpty() &&
-                 !mPlatform.IsEmpty() &&
-                 !mOscpu.IsEmpty(),
+                 !mLegacyAppVersion.IsEmpty(),
                  "HTTP cannot send practical requests without this much");
 
     // preallocate to worst-case size, which should always be better
     // than if we didn't preallocate at all.
     mUserAgent.SetCapacity(mLegacyAppName.Length() +
                            mLegacyAppVersion.Length() +
 #ifndef UA_SPARE_PLATFORM
                            mPlatform.Length() +
@@ -542,28 +540,29 @@ nsHttpHandler::BuildUserAgent()
     mUserAgent.Assign(mLegacyAppName);
     mUserAgent += '/';
     mUserAgent += mLegacyAppVersion;
     mUserAgent += ' ';
 
     // Application comment
     mUserAgent += '(';
 #ifndef UA_SPARE_PLATFORM
-    mUserAgent += mPlatform;
-    mUserAgent.AppendLiteral("; ");
+    if (!mPlatform.IsEmpty()) {
+      mUserAgent += mPlatform;
+      mUserAgent.AppendLiteral("; ");
+    }
 #endif
-#if defined(ANDROID) || defined(MOZ_PLATFORM_MAEMO)
     if (!mCompatDevice.IsEmpty()) {
         mUserAgent += mCompatDevice;
         mUserAgent.AppendLiteral("; ");
     }
-#else
-    mUserAgent += mOscpu;
-    mUserAgent.AppendLiteral("; ");
-#endif
+    else if (!mOscpu.IsEmpty()) {
+      mUserAgent += mOscpu;
+      mUserAgent.AppendLiteral("; ");
+    }
     mUserAgent += mMisc;
     mUserAgent += ')';
 
     // Product portion
     mUserAgent += ' ';
     mUserAgent += mProduct;
     mUserAgent += '/';
     mUserAgent += mProductSub;
@@ -586,47 +585,48 @@ nsHttpHandler::BuildUserAgent()
 #ifdef XP_WIN
 #define WNT_BASE "Windows NT %ld.%ld"
 #define W64_PREFIX "; Win64"
 #endif
 
 void
 nsHttpHandler::InitUserAgentComponents()
 {
+#ifndef MOZ_UA_OS_AGNOSTIC
     // Gather platform.
     mPlatform.AssignLiteral(
 #if defined(ANDROID)
     "Android"
 #elif defined(XP_OS2)
     "OS/2"
 #elif defined(XP_WIN)
     "Windows"
 #elif defined(XP_MACOSX)
     "Macintosh"
 #elif defined(MOZ_PLATFORM_MAEMO)
     "Maemo"
 #elif defined(MOZ_X11)
     "X11"
-#else
-    "?"
 #endif
     );
+#endif
 
 #if defined(ANDROID) || defined(MOZ_PLATFORM_MAEMO)
     nsCOMPtr<nsIPropertyBag2> infoService = do_GetService("@mozilla.org/system-info;1");
     NS_ASSERTION(infoService, "Could not find a system info service");
 
     bool isTablet = false;
     infoService->GetPropertyAsBool(NS_LITERAL_STRING("tablet"), &isTablet);
     if (isTablet)
         mCompatDevice.AssignLiteral("Tablet");
     else
         mCompatDevice.AssignLiteral("Mobile");
 #endif
 
+#ifndef MOZ_UA_OS_AGNOSTIC
     // Gather OS/CPU.
 #if defined(XP_OS2)
     ULONG os2ver = 0;
     DosQuerySysInfo(QSV_VERSION_MINOR, QSV_VERSION_MINOR,
                     &os2ver, sizeof(os2ver));
     if (os2ver == 11)
         mOscpu.AssignLiteral("2.11");
     else if (os2ver == 30)
@@ -702,16 +702,17 @@ nsHttpHandler::InitUserAgentComponents()
 #else
             buf += (char*)name.machine;
 #endif
         }
 
         mOscpu.Assign(buf);
     }
 #endif
+#endif
 
     mUserAgentIsDirty = true;
 }
 
 PRUint32
 nsHttpHandler::MaxSocketCount()
 {
     PR_CallOnce(&nsSocketTransportService::gMaxCountInitOnce,