autoContentFocus.uc.js

// ==UserScript==
// @name          autoContentFocus
// @namespace     http://d.hatena.ne.jp/murky-satyr
// @description   Focuses the content whenever a tab is selected.
// @include       main
// @compatibility Fx 3.0.*
// @version       2010-04-02
// ==/UserScript==
setTimeout(function(){
  gBrowser.tabContainer.addEventListener('TabSelect', function(e){
    var b = gBrowser.selectedTab.linkedBrowser;
    setTimeout(function acf(){
      var w = b.contentWindow;
      if(!w) return;
      w.focus();
      var e = document.commandDispatcher.focusedElement;
      if(e && /^(?:embed|object)$/i.test(e.nodeName)) e.blur();
    });
    // might as well...
    gPrefService.setBoolPref('accessibility.browsewithcaret', false);
  }, false);
});