Well, it actually did. Compare the Firefox 1 UI with current FF5 browser you will notice how much the UI has changes, simpler and cleaner, and the history-back buttons are gone.

FF1 http://en.wikipedia.org/wiki/File:Firefox_1_0_8.png

FF5 http://en.wikipedia.org/wiki/File:Firefox5.png

Remember Netscape? http://en.wikipedia.org/wiki/File:Netscape_Navigator.png

I do not think those pictures show what you think they do.

I can clearly see the back button in all 3 UI pictures, including the one for Firefox 5. For that matter, I can see the back button in the Firefox 5 window I'm using to type this comment. I agree that Firefox 5's UI is simpler and cleaner than Firefox 1's and Netscape's, but I don't know where you got the idea that they'd eliminated the back button.

Obviously you didn't read my reply clearly. I said "the history-back button" (one of back buttons)is gone, that is the little down-arrow button when clicked it shows the history.