Respectfully, I don’t have a solid answer to that question. How do users know that middle clicking on a link will open it in a new tab? That middle clicking a tab will close that tab?
And unlike in Safari, that right clicking the “Back” and “Forward” buttons will show the site history instead of showing “Customize Toolbar” option (thank you for implementing this, btw!). And when opening this list that middle clicking will open that specific site in a new tab?
Personally, I think it naturally flows with the perceived behavior of the middle click. Left clicking a link on the current page, a bookmark, or a link in the navigation history triggers an immediate change to the current active tab. A middle click on any of the above will open a new tab to the requested destination. In the case of the Reload button, left clicking it also triggers an immediate change in the current active tab, therefore I think it natural that a middle click would also follow the behavior as stated above - open a new tab to the requested destination, the same URL.
I hope this makes sense. Either way, I love what your team is doing! Keep up the great work!