--- a/toolkit/components/xulstore/XULStore.js
+++ b/toolkit/components/xulstore/XULStore.js
@@ -196,18 +196,24 @@ XULStore.prototype = {
     this.log("Saving " + attr + "=" + value + " for id=" + id + ", doc=" + docURI);
 
     if (!this._saveAllowed) {
       Services.console.logStringMessage("XULStore: Changes after profile-before-change are ignored!");
       return;
     }
 
     // bug 319846 -- don't save really long attributes or values.
-    if (id.length > 1024 || attr.length > 1024 || value.length > 1024)
-      throw Components.Exception("id, attribute, or value too long", Cr.NS_ERROR_ILLEGAL_VALUE);
+    if (id.length > 512 || attr.length > 512) {
+      throw Components.Exception("id or attribute name too long", Cr.NS_ERROR_ILLEGAL_VALUE);
+    }
+
+    if (value.length > 4096) {
+      Services.console.logStringMessage("XULStore: Warning, truncating long attribute value")
+      value = value.substr(0, 4096);
+    }
 
     let obj = this._data;
     if (!(docURI in obj)) {
       obj[docURI] = {};
     }
     obj = obj[docURI];
     if (!(id in obj)) {
       obj[id] = {};

--- a/toolkit/components/xulstore/tests/xpcshell/test_XULStore.js
+++ b/toolkit/components/xulstore/tests/xpcshell/test_XULStore.js
@@ -92,16 +92,31 @@ add_task(function* testImport(){
 
   yield OS.File.copy(src, dst);
 
   // Importing relies on XULStore not yet being loaded before this point.
   XULStore = Cc["@mozilla.org/xul/xulstore;1"].getService(Ci.nsIXULStore);
   checkOldStore();
 });
 
+add_task(function* testTruncation() {
+  let dos = Array(8192).join("~");
+  // Long id names should trigger an exception
+  Assert.throws(() => XULStore.setValue(browserURI, dos, "foo", "foo"), /NS_ERROR_ILLEGAL_VALUE/);
+
+  // Long attr names should trigger an exception
+  Assert.throws(() => XULStore.setValue(browserURI, "foo", dos, "foo"), /NS_ERROR_ILLEGAL_VALUE/);
+
+  // Long values should be truncated
+  XULStore.setValue(browserURI, "dos", "dos", dos);
+  dos =XULStore.getValue(browserURI, "dos", "dos");
+  do_check_true(dos.length == 4096)
+  XULStore.removeValue(browserURI, "dos", "dos")
+});
+
 add_task(function* testGetValue() {
   // Get non-existing property
   checkValue(browserURI, "side-window", "height", "");
 
   // Get existing property
   checkValue(browserURI, "main-window", "width", "994");
 });