fixes:

'tag' command works once more
  'source ~/' shows the matches
  fdopen checks whether the file exists, and throws a more-readable error if it doesn't
  'help' work if the current document is a XULDocument (it shows the help in a new tab)

chrome/content/vimperator/commands.js

@@ -356,7 +356,7 @@ var g_commands = [/*{{{*/
         ["tab"],
         ["tab {cmd}"],
         "Execute {cmd} and tell it to output in a new tab",
-        "Works for only commands that support it.",
+        "Works for only commands that support it.<br/>" +
         "Example: <code class=command>:tab help tab</code> opens the help in a new tab.",
         tab,
         null
@@ -1914,9 +1914,11 @@ function set(args, special)
 
 function source(filename, silent)
 {
+    if (!filename) return;
     try
     {
         var fd = fopen(filename, "<");
+        if (!fd) return;
         var s = fd.read();
         fd.close();
 

chrome/content/vimperator/completion.js

@@ -471,7 +471,7 @@ function get_file_completions(filter)/*{{{*/
         return [[path], ''];
     });
 
-    return g_completions = build_longest_starting_substring(mapped, filter);
+    return g_completions = build_longest_starting_substring(mapped, new_filter);
 }/*}}}*/
 
 function get_help_completions(filter)/*{{{*/

chrome/content/vimperator/file.js

@@ -87,10 +87,14 @@ function LocalFile(file, mode, perms, tmp)
     {
         this.localFile = classes[LOCALFILE_CTRID].createInstance(nsILocalFile);
         this.localFile.initWithPath(file);
+        if (!this.localFile.exists())
+            throw "No such file or directory";
     }
     else if (file instanceof nsILocalFile)
     {
         this.localFile = file;
+        if (!this.localFile.exists())
+            throw "No such file or directory";
     }
     else
     {

chrome/content/vimperator/help.js

@@ -33,7 +33,7 @@ function help(section, easter)
         echoerr("E478: Don't panic!");
         return;
     }
-    if (arguments[3] && arguments[3].inTab)
+    if ((arguments[3] && arguments[3].inTab) || !window.content.document.open)
         openURLsInNewTab("", true); 
 
     var doc = window.content.document;