This is a small usability idea to help users not to close the wrong tab.
With more and more tabs open, it's not easy to see which one is a new tab and which one is a tab with previous navigation. Typically some links opens in new tabs and some links on the same tab. In the latter, it can result in closing the tab instead of going back to the previous page. Looking toward the big back button as an indicator (in the main toolbar) is not enough as human tends to decide too quickly.
I suggest to add an indicator or a small back button (small as the close button) on tabs with previous navigation url.