It does exactly what it says in the description. And it works great for common use cases. However, it doesn't suit me. I currently have 473 tabs in the current window (most of them are discarded and hidden). To manage so many tabs, I use this group tabs extension (https://addons.mozilla.org/en-US/firefox/addon/power-tabs/). So, when using these two addons together, it is expected that a new tab should be opened every time there are no VISIBLE (not hidden) NOT PINNED tabs on the tabbar. For now the focus just goes to the rightmost pinned tab. Adding this feature would be quite justified, as the user community of certain group-tabs extensions is quite large

Если открыть пустую вкладку и нажать на неё пинед(pined), то браузер не будет закрываться.
if you open an empty tab and click on it (pinned), the browser will not close.