This morning, Firefox was updated to the latest UI. So far today, I've mistakenly clicked on "close tab" 5-6 times when I intended to open a new tab instead, because there no longer is a visible + and the X looks like one.
I just use mouse 3 somewhere to the right of the + when I want a new tab without clicking on a URL unless I somehow have 6 trillion tabs already open, as that automatically opens a new tab.