Earlier quoted context omitted.
Meh the + is on the same spot as the tabs symbol so it's just a fast double tap and that means one less button in the interface. Edit: OK, the fact that you have to wait for the animation is annoying indeed.
Yup. Even though in Firefox Android 'regular' that's at the same page, as you noted it's not always that fast, especially for low end phones. But with this new Firefox Preview even the Plus button is not at the same 'position', and the new page opening up with animation takes even more time.
At the time you opened the tab menu, you're already in a new tab.