--- a/toolkit/components/processsingleton/ContentProcessSingleton.js
+++ b/toolkit/components/processsingleton/ContentProcessSingleton.js
@@ -9,16 +9,37 @@ const Ci = Components.interfaces;
 
 Cu.import("resource://gre/modules/Services.jsm");
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 
 XPCOMUtils.defineLazyServiceGetter(this, "cpmm",
                                    "@mozilla.org/childprocessmessagemanager;1",
                                    "nsIMessageSender");
 
+/*
+ * The message manager has an upper limit on message sizes that it can
+ * reliably forward to the parent so we limit the size of console log event
+ * messages that we forward here. The web console is local and receives the
+ * full console message, but addons subscribed to console event messages
+ * in the parent receive the truncated version. Due to fragmentation,
+ * messages as small as 1MB have resulted in IPC allocation failures on
+ * 32-bit platforms. To limit IPC allocation sizes, console.log messages
+ * with arguments with total size > MSG_MGR_CONSOLE_MAX_SIZE (bytes) have
+ * their arguments completely truncated. MSG_MGR_CONSOLE_VAR_SIZE is an
+ * approximation of how much space (in bytes) a JS non-string variable will
+ * require in the manager's implementation. For strings, we use 2 bytes per
+ * char. The console message URI and function name are limited to
+ * MSG_MGR_CONSOLE_INFO_MAX characters. We don't attempt to calculate
+ * the exact amount of space the message manager implementation will require
+ * for a given message so this is imperfect.
+ */
+const MSG_MGR_CONSOLE_MAX_SIZE = 1024 * 1024; // 1MB
+const MSG_MGR_CONSOLE_VAR_SIZE = 8;
+const MSG_MGR_CONSOLE_INFO_MAX = 1024;
+
 function ContentProcessSingleton() {}
 ContentProcessSingleton.prototype = {
   classID: Components.ID("{ca2a8470-45c7-11e4-916c-0800200c9a66}"),
   QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver,
                                          Ci.nsISupportsWeakReference]),
 
   observe: function(subject, topic, data) {
     switch (topic) {
@@ -28,30 +49,51 @@ ContentProcessSingleton.prototype = {
       cpmm.addMessageListener("DevTools:InitDebuggerServer", this);
       break;
     }
     case "console-api-log-event": {
       let consoleMsg = subject.wrappedJSObject;
 
       let msgData = {
         level: consoleMsg.level,
-        filename: consoleMsg.filename,
+        filename: consoleMsg.filename.substring(0, MSG_MGR_CONSOLE_INFO_MAX),
         lineNumber: consoleMsg.lineNumber,
-        functionName: consoleMsg.functionName,
+        functionName: consoleMsg.functionName.substring(0,
+          MSG_MGR_CONSOLE_INFO_MAX),
         timeStamp: consoleMsg.timeStamp,
         arguments: [],
       };
 
       // We can't send objects over the message manager, so we sanitize
-      // them out.
+      // them out, replacing those arguments with "<unavailable>".
+      let unavailString = "<unavailable>";
+      let unavailStringLength = unavailString.length * 2; // 2-bytes per char
+
+      // When the sum of argument sizes reaches MSG_MGR_CONSOLE_MAX_SIZE,
+      // replace all arguments with "<truncated>".
+      let totalArgLength = 0;
+
+      // Walk through the arguments, checking the type and size.
       for (let arg of consoleMsg.arguments) {
-        if ((typeof arg == "object" || typeof arg == "function") && arg !== null) {
-          msgData.arguments.push("<unavailable>");
+        if ((typeof arg == "object" || typeof arg == "function") &&
+            arg !== null) {
+          arg = unavailString;
+          totalArgLength += unavailStringLength;
+        } else if (typeof arg == "string") {
+          totalArgLength += arg.length * 2; // 2-bytes per char
         } else {
+          totalArgLength += MSG_MGR_CONSOLE_VAR_SIZE;
+        }
+
+        if (totalArgLength <= MSG_MGR_CONSOLE_MAX_SIZE) {
           msgData.arguments.push(arg);
+        } else {
+          // arguments take up too much space
+          msgData.arguments = ["<truncated>"];
+          break;
         }
       }
 
       cpmm.sendAsyncMessage("Console:Log", msgData);
       break;
     }
 
     case "xpcom-shutdown":