The line at the top of the inactive tab breaks what users understand from so many other modern tab patterns.
Material uses a line on the active tab, though below the label. The line is shorthand for "this is active". Users thus think that whichever tab has a line on it is the active one.
Older users, such as myself, remember UIs like this one, where inactive items were "beveled" to look more clickable, while active elements were made to look "depressed". I suspect Firefox is trying that idea.
If that's the case, then they should be consistent and make the "plus" button next to the last tab also beveled, rather than combining beveled and flat clickable elements.