ie:
document.Save(document.FullName);
try{
wnd = shell.FindWindow("IEFrame","");
wnd.SetForeground();
shell.SendKeys( "{F5}" );
Sleep(1000);
wnd2 = shell.FindWindow("EmEditorMainFrame3","");
wnd2.SetForeground();
}catch(e){
WshShell = new ActiveXObject( "WScript.Shell" );
WshShell.Run('iexplore.exe '+document.FullName);
}
firefox:
document.Save(document.FullName);
try{
wnd = shell.FindWindow( "MozillaUIWindowClass", "" );
wnd.SetForeground();
shell.SendKeys( "{F5}" );
Sleep(1000)
wnd2 = shell.FindWindow("EmEditorMainFrame3","");
wnd2.SetForeground();
}catch(e){
WshShell = new ActiveXObject( "WScript.Shell" );
WshShell.Run('firefox.exe '+document.FullName);
}

I use sinppets, set up trigger "ie" or "ff".