Javascript get custom button's text value

You can do that through the textContent/innerText properties (browser-dependant). Here's an example that will work no matter which property the browser uses:

var elem = document.getElementById('ext-gen26');
var txt = elem.textContent || elem.innerText;
alert(txt);

http://jsfiddle.net/ThiefMaster/EcMRT/

You could also do it using jQuery:

alert($('#ext-gen26').text());

If you're trying to locate the button entirely by its text content, I'd grab a list of all buttons and loop through them to find this one:

function findButtonbyTextContent(text) {
  var buttons = document.querySelectorAll('button');
  for (var i=0, l=buttons.length; i<l; i++) {
    if (buttons[i].firstChild.nodeValue == text)
      return buttons[i];
  }  
}

Of course, if the content of this button changes even a little your code will need to be updated.