This add-on does not seem to work with the most recent version of firefox. Even if it did, the lack of a feature to replace specific elements on the HTML document itself would've still not made it great as an add-on. Apparently, you can only add your own html code at the top of the document, at the bottom of the document or replace the whole document with your code which is far from ideal, assuming it even worked to begin with.

You would rather stick to a script manager or a stylesheet manager instead of this add-on.