Currently when you close the last tab Orion will close the main window. And people, I included, want this window to remain open with a new fresh start page.
Adding a checkbox at "Orion > Settings > Tabs" named "Close window when closing last tab"
Behaviour:
When the checkbox is checked (true): Orion main window closes upon closing last tab (current behaviour)
When the checkbos is unchecked (false): Orion main window will not close upon closing last tab. But rather open a "new tab" with the "Start Page". (Similar behaviour on mobile Orion / mobile Safari).