diff --git a/embedding/browser/activex/src/control/WebBrowserContainer.cpp b/embedding/browser/activex/src/control/WebBrowserContainer.cpp
index 20e96fee5dee..3de5c803aef9 100644
--- a/embedding/browser/activex/src/control/WebBrowserContainer.cpp
+++ b/embedding/browser/activex/src/control/WebBrowserContainer.cpp
@@ -401,11 +401,14 @@ NS_IMETHODIMP CWebBrowserContainer::IsPreferred(const char *aContentType, char *
 /* boolean canHandleContent (in string aContentType, in PRBool aIsContentPreferred, out string aDesiredContentType); */
 NS_IMETHODIMP CWebBrowserContainer::CanHandleContent(const char *aContentType, PRBool aIsContentPreferred, char **aDesiredContentType, PRBool *_retval)
 {
+    *_retval = PR_FALSE;
+    
     if (aContentType)
     {
         nsCOMPtr<nsICategoryManager> catMgr;
         nsresult rv;
         catMgr = do_GetService(NS_CATEGORYMANAGER_CONTRACTID, &rv);
+        NS_ENSURE_SUCCESS(rv, rv);
         nsXPIDLCString value;
         rv = catMgr->GetCategoryEntry("Gecko-Content-Viewers",
             aContentType, 
diff --git a/embedding/browser/gtk/src/EmbedContentListener.cpp b/embedding/browser/gtk/src/EmbedContentListener.cpp
index 912c8e718dc6..4f2c3e576025 100644
--- a/embedding/browser/gtk/src/EmbedContentListener.cpp
+++ b/embedding/browser/gtk/src/EmbedContentListener.cpp
@@ -27,6 +27,9 @@
 #include "EmbedContentListener.h"
 #include "EmbedPrivate.h"
 
+#include "nsICategoryManager.h"
+#include "nsIServiceManagerUtils.h"
+
 EmbedContentListener::EmbedContentListener(void)
 {
   mOwner = nsnull;
@@ -84,27 +87,8 @@ EmbedContentListener::IsPreferred(const char        *aContentType,
 				  char             **aDesiredContentType,
 				  PRBool            *aCanHandleContent)
 {
-  if (aContentType &&
-      (!strcasecmp(aContentType, "text/html")   ||
-       !strcasecmp(aContentType, "text/plain")  ||
-       !strcasecmp(aContentType, "application/vnd.mozilla.xul+xml")    ||
-       !strcasecmp(aContentType, "text/rdf")    ||
-       !strcasecmp(aContentType, "text/xml")    ||
-       !strcasecmp(aContentType, "application/xml")    ||
-       !strcasecmp(aContentType, "application/xhtml+xml")    ||
-       !strcasecmp(aContentType, "text/css")    ||
-       !strcasecmp(aContentType, "image/gif")   ||
-       !strcasecmp(aContentType, "image/jpeg")  ||
-       !strcasecmp(aContentType, "image/png")   ||
-       !strcasecmp(aContentType, "image/tiff")  ||
-       !strcasecmp(aContentType, "application/http-index-format"))) {
-    *aCanHandleContent = PR_TRUE;
-  }
-  else {
-    *aCanHandleContent = PR_FALSE;
-  }
-
-  return NS_OK;
+  return CanHandleContent(aContentType, PR_TRUE, aDesiredContentType,
+			  aCanHandleContent);
 }
 
 NS_IMETHODIMP
@@ -113,7 +97,29 @@ EmbedContentListener::CanHandleContent(const char        *aContentType,
 				       char             **aDesiredContentType,
 				       PRBool            *_retval)
 {
-  return NS_ERROR_NOT_IMPLEMENTED;
+  *_retval = PR_FALSE;
+  
+  if (aContentType) {
+    nsCOMPtr<nsICategoryManager> catMgr;
+    nsresult rv;
+    catMgr = do_GetService("@mozilla.org/categorymanager;1", &rv);
+    NS_ENSURE_SUCCESS(rv, rv);
+    nsXPIDLCString value;
+    rv = catMgr->GetCategoryEntry("Gecko-Content-Viewers",
+				  aContentType, 
+				  getter_Copies(value));
+
+    // If the category manager can't find what we're looking for
+    // it returns NS_ERROR_NOT_AVAILABLE, we don't want to propagate
+    // that to the caller since it's really not a failure
+
+    if (NS_FAILED(rv) && rv != NS_ERROR_NOT_AVAILABLE)
+      return rv;
+
+    if (value && *value)
+      *_retval = PR_TRUE;
+  }
+  return NS_OK;
 }
 
 NS_IMETHODIMP
diff --git a/embedding/browser/photon/src/EmbedContentListener.cpp b/embedding/browser/photon/src/EmbedContentListener.cpp
index af30dee06792..d61f940dae15 100644
--- a/embedding/browser/photon/src/EmbedContentListener.cpp
+++ b/embedding/browser/photon/src/EmbedContentListener.cpp
@@ -30,9 +30,11 @@
 
 #include "PtMozilla.h"
 
+#include "nsICategoryManager.h"
+#include "nsIServiceManagerUtils.h"
+
 EmbedContentListener::EmbedContentListener(void)
 {
-	NS_INIT_ISUPPORTS();
   mOwner = nsnull;
 }
 
@@ -97,27 +99,8 @@ EmbedContentListener::IsPreferred(const char        *aContentType,
 				  char             **aDesiredContentType,
 				  PRBool            *aCanHandleContent)
 {
-  if (aContentType &&
-      (!strcasecmp(aContentType, "text/html")   ||
-       !strcasecmp(aContentType, "text/plain")  ||
-       !strcasecmp(aContentType, "application/vnd.mozilla.xul+xml")    ||
-       !strcasecmp(aContentType, "text/rdf")    ||
-       !strcasecmp(aContentType, "text/xml")    ||
-       !strcasecmp(aContentType, "application/xml")    ||
-       !strcasecmp(aContentType, "application/xhtml+xml")    ||
-       !strcasecmp(aContentType, "text/css")    ||
-       !strcasecmp(aContentType, "image/gif")   ||
-       !strcasecmp(aContentType, "image/jpeg")  ||
-       !strcasecmp(aContentType, "image/png")   ||
-       !strcasecmp(aContentType, "image/tiff")  ||
-       !strcasecmp(aContentType, "application/http-index-format"))) {
-    *aCanHandleContent = PR_TRUE;
-  }
-  else {
-    *aCanHandleContent = PR_FALSE;
-  }
-
-  return NS_OK;
+  return CanHandleContent(aContentType, PR_TRUE, aDesiredContentType,
+			  aCanHandleContent);
 }
 
 NS_IMETHODIMP
@@ -126,7 +109,29 @@ EmbedContentListener::CanHandleContent(const char        *aContentType,
 				       char             **aDesiredContentType,
 				       PRBool            *_retval)
 {
-  return NS_ERROR_NOT_IMPLEMENTED;
+  *_retval = PR_FALSE;
+  
+  if (aContentType) {
+    nsCOMPtr<nsICategoryManager> catMgr;
+    nsresult rv;
+    catMgr = do_GetService("@mozilla.org/categorymanager;1", &rv);
+    NS_ENSURE_SUCCESS(rv, rv);
+    nsXPIDLCString value;
+    rv = catMgr->GetCategoryEntry("Gecko-Content-Viewers",
+				  aContentType, 
+				  getter_Copies(value));
+
+    // If the category manager can't find what we're looking for
+    // it returns NS_ERROR_NOT_AVAILABLE, we don't want to propagate
+    // that to the caller since it's really not a failure
+
+    if (NS_FAILED(rv) && rv != NS_ERROR_NOT_AVAILABLE)
+      return rv;
+
+    if (value && *value)
+      *_retval = PR_TRUE;
+  }
+  return NS_OK;
 }
 
 NS_IMETHODIMP
diff --git a/xpfe/browser/resources/content/nsBrowserContentListener.js b/xpfe/browser/resources/content/nsBrowserContentListener.js
index f716852bf8db..c01b3628d203 100644
--- a/xpfe/browser/resources/content/nsBrowserContentListener.js
+++ b/xpfe/browser/resources/content/nsBrowserContentListener.js
@@ -130,21 +130,21 @@ nsBrowserContentListener.prototype =
 
     isPreferred: function(contentType, desiredContentType)
     {
-        // seems like we should be getting this from helper apps or something
-        switch(contentType) {
-            case "text/html":
-            case "text/xul":
-            case "text/rdf":
-            case "text/xml":
-            case "text/css":
-            case "image/gif":
-            case "image/jpeg":
-            case "image/png":
-            case "text/plain":
-            case "application/http-index-format":
+        try {
+            var catMgr = Components.classes["@mozilla.org/categorymanager;1"]
+                        .getService(Components.interfaces.nsICategoryManager);
+            var entry = catMgr.getCategoryEntry("Gecko-Content-Viewers",
+                                                contentType);
+            if (entry) {
                 return true;
+            }
+
+            return false;
+        } catch (e) {
+            // XXX propagate failures other than "NS_ERROR_NOT_AVAILABLE"?
+            // This seems to never get called, so not like it matters....
+            return false;
         }
-        return false;
     },
     canHandleContent: function(contentType, isContentPreferred, desiredContentType)
     {