How to make fennec(firefox) not reopen and automatically load the last open tab?
To disable restorng tabs, go to Firefox settings, then Advanced, click on Restore tabs. Select "Don't restore after quitting Firefox". It will not open previous tabs when you restart the browser.

But you said that is not the behavior you are looking for. Then the other possibility is to set as homepage a page you don't care for.  Go to Settings -> General -> Home -> Set a homepage, then select Custom and enter: about:blank (a blank page as your homepage). Click OK. Enable "Also use for new tabs".

From now, before closing Firefox, open a new tab. A blank page will be displayed. When you open the browser again, the blank page will be loaded. The other tabs will still be there.