diff --git a/netwerk/protocol/http/HttpBaseChannel.cpp b/netwerk/protocol/http/HttpBaseChannel.cpp
index c430ad085..183684b68 100644
--- a/netwerk/protocol/http/HttpBaseChannel.cpp
+++ b/netwerk/protocol/http/HttpBaseChannel.cpp
@@ -181,7 +181,8 @@ HttpBaseChannel::Init(nsIURI *aURI,
   rv = mRequestHead.SetHeader(nsHttp::Host, hostLine);
   if (NS_FAILED(rv)) return rv;
 
-  rv = gHttpHandler->AddStandardRequestHeaders(&mRequestHead.Headers(), isHTTPS);
+  rv = gHttpHandler->AddStandardRequestHeaders(&mRequestHead.Headers(), isHTTPS,
+hostLine);
   if (NS_FAILED(rv)) return rv;
 
   nsAutoCString type;
diff --git a/netwerk/protocol/http/nsHttpHandler.cpp b/netwerk/protocol/http/nsHttpHandler.cpp
index 7c50cfe94..d118a488d 100644
--- a/netwerk/protocol/http/nsHttpHandler.cpp
+++ b/netwerk/protocol/http/nsHttpHandler.cpp
@@ -422,14 +422,28 @@ nsHttpHandler::InitConnectionMgr()
 }
 
 nsresult
-nsHttpHandler::AddStandardRequestHeaders(nsHttpHeaderArray *request, bool isSecure)
+nsHttpHandler::AddStandardRequestHeaders(nsHttpHeaderArray *request,
+                                         bool isSecure,
+                                         const nsACString &hostLine)
 {
     nsresult rv;
 
-    // Add the "User-Agent" header
-    rv = request->SetHeader(nsHttp::User_Agent, UserAgent(),
-                            false, nsHttpHeaderArray::eVarietyDefault);
-    if (NS_FAILED(rv)) return rv;
+    // Add the "User-Agent" header (unless we have blacklisted this site and
+    // we aren't using a custom user agent; see TenFourFox issue 422).
+    if (mUserAgentOverride || (
+            !hostLine.EqualsLiteral("i.imgur.com") &&
+            !hostLine.EqualsLiteral("imgur.com") &&
+    1)) {
+        rv = request->SetHeader(nsHttp::User_Agent, UserAgent(),
+                                false, nsHttpHeaderArray::eVarietyDefault);
+        if (NS_FAILED(rv)) return rv;
+    }
+#if DEBUG
+    else {
+        fprintf(stderr, "User agent header suppressed for host: %s\n",
+                PromiseFlatCString(hostLine).get());
+    }
+#endif
 
     // MIME based content negotiation lives!
     // Add the "Accept" header.  Note, this is set as an override because the
diff --git a/netwerk/protocol/http/nsHttpHandler.h b/netwerk/protocol/http/nsHttpHandler.h
index 6bd27389e..0e825700c 100644
--- a/netwerk/protocol/http/nsHttpHandler.h
+++ b/netwerk/protocol/http/nsHttpHandler.h
@@ -67,7 +67,8 @@ public:
     nsHttpHandler();
 
     nsresult Init();
-    nsresult AddStandardRequestHeaders(nsHttpHeaderArray *, bool isSecure);
+    nsresult AddStandardRequestHeaders(nsHttpHeaderArray *, bool isSecure,
+                                       const nsACString &hostLine);
     nsresult AddConnectionHeader(nsHttpHeaderArray *,
                                  uint32_t capabilities);
     bool     IsAcceptableEncoding(const char *encoding, bool isSecure);