Many have asked about that. It's not a bug.
I have just been asked to update Firefox on my desktop, so now have 15.0.1 and using Windows XP
I now do not have a 'page back' button!
On my lap top, also XP and 15.0.1 I have a page back button. On the desk top, same version as I said, I have 2 buttons to the left of the URL address line. These are 'Display pages you have recently visited' and 'display your bookmarks'. This is where the back button should be.
Have I done something wrong? I dont see how, as I have just done a standard install.
Might have to go back to IE!!!