I had the same problem, to get it back you just need to right click on the tab bar, click customise, then drag the new tab plus sign symbol to the end of the tab bar. When you click done it will attach itself on to the end of the last tab as before.

thank you much. that was taking so much of my time trying to figure out something that small but I was so used to it

there is no new tab sign in the customize menu...where did it go? I just updated to the latest firefox, but this didn't happen with Firefox 3.+

See [/questions/796580]