Do Not Auto Activate Pinned Tabs от Alex Z.

Note! This addon was rewritten using WebExtensions and now compatible with FF57+. But this can also lead to some issues in Firefox 53-56 while the whole FF ecosystem in transition state now. If you are experiencing any issues you can downgrade or temporarily disable this extension until WebExtensions API will be stabilized.

Before installing - if you close last normal (non-pinned) tab then rightmost pinned tab is automatically activated.
After installing - new tab will be created instead of showing pinned tab.

If you have any questions or suggestions, please feel free to contact me:
shoomyth at google's mail (some conspiracy, if you understand :))