Add some weak support for _main until I can hack the C++ to fully support it like _content.

browser/base/content/browser.js

@@ -3823,7 +3823,9 @@ function asyncOpenWebPanel(event)
    if (linkNode) {
      if (fieldNormalClicks && event.button == 0 && !event.ctrlKey && !event.shiftKey && 
          !event.altKey && !event.metaKey && 
-         (!linkNode.getAttribute("target") || linkNode.getAttribute("target") == "_content")) {
+         (!linkNode.getAttribute("target") || linkNode.getAttribute("target") == "_content") ||
+          linkNode.getAttribute("target") == "_main") // IE uses _main, SeaMonkey uses _content, we support both
+     {
        var url = getShortcutOrURI(linkNode.href);
        if (!url)
          return false;