In the previous verison of xfce4-terminal, I could stop it from intercepting Alt+1 to Alt+9 (which are currently bound to tab switching). I'd go to "Preferences..." -> "Advanced" tab -> "Disable all menu access keys (such as Alt+f)" checkbox, and check it. This stopped working after upgrading to 0.6.0. (I noticed that it doesn't accept these keys if I have no other tabs open in the terminal. Is this a clue as to why this stopped working?) Thanks!
Oops, typo: "(I noticed that it doesn't INTERCEPT these keys if I have no other tabs open.)"
Enable editable menu accelerators, open a bunch of tab and press backspace while hoverting the Alt+X menu items.