There are more doubly encoded characters than just that one. PM's cookie is weird… In any case, I did this to fix it instead:

--- pmchatter.html.old  2004-12-20 02:49:00.000000000 +0100
+++ pmchatter.html      2004-12-20 02:49:03.000000000 +0100
@@ -34,3 +34,3 @@
 
-               try{ cookie.parse( this.req.getResponseHeader( 'Set-Co
+okie' ) ); }
+               try{ decodeURIComponent( cookie.parse( this.req.getRes
+ponseHeader( 'Set-Cookie' ) ) ); }
                catch( e ) { }
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