Heads up, I haven't forgotten about this extension! I just haven't had time to work on it. Stay tuned though, I am planning to release an update by the end of the year.
The default new tab page of Firefox does only provide a limited amount of information. This addon consequently provides a replacement by providing an overview of the bookmarks instead. In doing so, the layout as well as the style can easily be modified such that a wide variety of changes can be made to customize the addon appropriately.
Why is the addon not being shown when a new tab is being opened? Try to open the addon through the options button within the extension overview of Firefox. Subsequently open the settings menu of the addon through the gearwheel on the lower right and make sure that the first checkbox is set.
Is it possible to have multiple columns? It is indeed possible to customize the entire layout through the incorporation of up to three columns. In order to do so, open the layout manager of the addon through the arrows on the lower right.
Submit Bugs and Requests: Github