Copy to clipboard not working in Firefox 53 (OSX)

Question

I am attempting to copy text content from an iframe using javascript. I have code almost identical to the accepted answer for this question How can I copy text in one click in Mozilla Firefox? however it no longer seems to work.

It works fine on Safari, Chrome and IE. In FF the selecting works fine, but I am getting the following error on the console:

document.execCommand(‘cut’/‘copy’) was denied because it was not called from inside a short running user-generated event handler.

The problem is, just like the accepted answer in the question above, I am in a user-generated event handler.

In recent versions of FF clipboard events should available by default, I have checked that they are enabled but no joy.

Has anyone encountered any issues like this recently?

EDIT

Adding code snippet. Its a little ugly due to iframe

     (function() {
        var el = document.getElementById("copyButton");
        el.addEventListener("click", selectAllText, false);
    })();

function selectAllText(){

    var window;
    // Google Chrome Iframe
    if(noteViewFrame.contentWindow){
        window = noteViewFrame.contentWindow;
    }else {
        // Other browsers
        window = noteViewFrame;
    }
    window.makeSelectable();
    var node = window.document.getElementById( 'NoteEditor_0' );

    if ( window.document.selection ) {
        var range = window.document.body.createTextRange();
        range.moveToElementText( node  );
        range.select();
    } else if ( window.getSelection ) {
        var range = window.document.createRange();
        range.selectNodeContents( node );
        window.getSelection().removeAllRanges();
        window.getSelection().addRange( range );
    }

    window.document.execCommand('copy');
}

Event is triggered, contents are selected, but on FF the above error is shown in the console