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diff --git a/xpcom/base/nsIVersionComparator.idl b/xpcom/base/nsIVersionComparator.idl
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+++ b/xpcom/base/nsIVersionComparator.idl
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+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+/**
+ * Version strings are dot-separated sequences of version-parts.
+ *
+ * A version-part consists of up to four parts, all of which are optional:
+ *
+ * <number-a><string-b><number-c><string-d (everything else)>
+ *
+ * A version-part may also consist of a single asterisk "*" which indicates
+ * "infinity".
+ *
+ * Numbers are base-10, and are zero if left out.
+ * Strings are compared bytewise.
+ *
+ * For additional backwards compatibility, if "string-b" is "+" then
+ * "number-a" is incremented by 1 and "string-b" becomes "pre".
+ *
+ * 1.0pre1
+ * < 1.0pre2
+ *   < 1.0 == 1.0.0 == 1.0.0.0
+ *     < 1.1pre == 1.1pre0 == 1.0+
+ *       < 1.1pre1a
+ *         < 1.1pre1
+ *           < 1.1pre10a
+ *             < 1.1pre10
+ *
+ * Although not required by this interface, it is recommended that
+ * numbers remain within the limits of a signed char, i.e. -127 to 128.
+ */
+[builtinclass, scriptable, uuid(e6cd620a-edbb-41d2-9e42-9a2ffc8107f3)]
+interface nsIVersionComparator : nsISupports
+{
+  /**
+   * Compare two version strings
+   * @param   A   The first version
+   * @param   B   The second version
+   * @returns < 0 if A < B
+   *          = 0 if A == B
+   *          > 0 if A > B
+   */
+  long compare(in ACString A, in ACString B);
+};