This must have come in with RC1's changes to the context menus. If one selects some text on a page and then right-clicks, the resulting menu does not have "Back" on it. While it makes some sense to limit the actions to what one can do with highlighted text, I find this very annoying since using "Find" leaves text highlighted and I tend to navigate with the context menus.
Related: bug 135331, bug 136665 - both invalid/wontfix. Resolving as invalid since the menu items were deliberatly removed: see bug 108099 and bug 75338.