Snippet updates meta values to undefined in FireFox

In FF when using the snippet the metadata and the snippet get updated with "undefined" strings. (See Screenshot)

This is because FF doesn't support the use of innerText (yet) which is used to update the snippet and matching metadata fields.

Attached you can find a patch file which checks if innerText returns undefined, that's when we use the w3c compatible textContent.

Information about textContent: https://developer.mozilla.org/en-US/docs/Web/API/Node/textContent

textContent is not compatible with older versions of IE so I have left the support for innerText as well.