diff --git a/netwerk/protocol/http/nsHttpHandler.cpp b/netwerk/protocol/http/nsHttpHandler.cpp
index 2691a2a77..c88499267 100644
--- a/netwerk/protocol/http/nsHttpHandler.cpp
+++ b/netwerk/protocol/http/nsHttpHandler.cpp
@@ -334,28 +334,9 @@ nsHttpHandler::Init()
         }
         mAppName.StripChars(R"( ()<>@,;:\"/[]?={})");
     }
-    
-    nsCString dynamicBuildID;
-    if (appInfo) {
-      appInfo->GetPlatformBuildID(dynamicBuildID);
-      if (dynamicBuildID.Length() > 8 )
-        dynamicBuildID.Left(dynamicBuildID, 8);
-    }
 
-    if (mAppVersionIsBuildID) {
-      // Override BuildID
-      mAppVersion.AssignLiteral(MOZ_UA_BUILDID);
-    } else if (appInfo) {
-      appInfo->GetVersion(mAppVersion);
-    } else {
-      // Fall back to platform if appInfo is unavailable
-      mAppVersion.AssignLiteral(MOZILLA_UAVERSION);
-    }
+    BuildAppVersion();    
 
-    // If there's no override set, set it to the dynamic BuildID
-    if (mAppVersion.IsEmpty())
-      mAppVersion.Assign(dynamicBuildID);
-        
     mSessionStartTime = NowInSeconds();
     mHandlerActive = true;
 
@@ -374,8 +355,15 @@ nsHttpHandler::Init()
     // Goanna slice version
     mProductSub.AssignLiteral(MOZILLA_UAVERSION);
     
-    if (mProductSub.IsEmpty())
-        mProductSub.Assign(dynamicBuildID);
+    if (mProductSub.IsEmpty()) {
+      nsCString dynamicBuildID;
+      if (appInfo) {
+        appInfo->GetPlatformBuildID(dynamicBuildID);
+        if (dynamicBuildID.Length() > 8 )
+          dynamicBuildID.Left(dynamicBuildID, 8);
+      }
+      mProductSub.Assign(dynamicBuildID);
+    }
 
 #if DEBUG
     // dump user agent prefs
@@ -681,6 +669,37 @@ nsHttpHandler::GenerateHostPort(const nsCString& host, int32_t port,
 // nsHttpHandler <private>
 //-----------------------------------------------------------------------------
 
+void
+nsHttpHandler::BuildAppVersion()
+{
+    nsCOMPtr<nsIXULAppInfo> appInfo = do_GetService("@mozilla.org/xre/app-info;1");
+
+    nsCString dynamicBuildID;
+    if (appInfo) {
+      appInfo->GetPlatformBuildID(dynamicBuildID);
+      if (dynamicBuildID.Length() > 8 )
+        dynamicBuildID.Left(dynamicBuildID, 8);
+    }
+
+    if (mAppVersionIsBuildID) {
+      // Override BuildID
+      mAppVersion.Assign(dynamicBuildID);
+    } else if (appInfo) {
+      appInfo->GetVersion(mAppVersion);
+    } else {
+      // Fall back to platform if appInfo is unavailable
+      mAppVersion.AssignLiteral(MOZILLA_UAVERSION);
+    }
+
+    // If there's still no version set, set it to a fixed BuildID
+    if (mAppVersion.IsEmpty()) {
+      mAppVersion.AssignLiteral(MOZ_UA_BUILDID);
+    }
+    if (mAppVersion.IsEmpty()) {
+      mAppVersion.AssignLiteral("20200101");
+    }
+}
+
 const nsAFlatCString &
 nsHttpHandler::UserAgent()
 {
@@ -851,21 +870,6 @@ nsHttpHandler::InitUserAgentComponents()
         mDeviceModelId = mozilla::net::GetDeviceModelId();
     }
 #endif // ANDROID
-
-#ifdef MOZ_MULET
-    {
-        // Add the `Mobile` or `Tablet` or `TV` token when running in the b2g
-        // desktop simulator via preference.
-        nsCString deviceType;
-        nsresult rv = Preferences::GetCString("devtools.useragent.device_type", &deviceType);
-        if (NS_SUCCEEDED(rv)) {
-            mCompatDevice.Assign(deviceType);
-        } else {
-            mCompatDevice.AssignLiteral("Mobile");
-        }
-    }
-#endif // MOZ_MULET
-
 #ifndef MOZ_UA_OS_AGNOSTIC
     // Gather OS/CPU.
 #if defined(XP_WIN)
@@ -993,6 +997,10 @@ nsHttpHandler::PrefsChanged(nsIPrefBranch *prefs, const char *pref)
     if (PREF_CHANGED(UA_PREF("appVersionIsBuildID"))) {
         rv = prefs->GetBoolPref(UA_PREF("appVersionIsBuildID"), &cVar);
         mAppVersionIsBuildID = (NS_SUCCEEDED(rv) && cVar);
+        
+        // Rebuild application version string.
+        BuildAppVersion();
+
         mUserAgentIsDirty = true;
     }
 
diff --git a/netwerk/protocol/http/nsHttpHandler.h b/netwerk/protocol/http/nsHttpHandler.h
index 402147577..36fda6ace 100644
--- a/netwerk/protocol/http/nsHttpHandler.h
+++ b/netwerk/protocol/http/nsHttpHandler.h
@@ -391,6 +391,7 @@ private:
     //
     // Useragent/prefs helper methods
     //
+    void     BuildAppVersion();
     void     BuildUserAgent();
     void     InitUserAgentComponents();
     void     PrefsChanged(nsIPrefBranch *prefs, const char *pref);