The new /home was created within the new / partition, which is not what I want: 1) I want to keep the partitions separate, for future upgrades; and 2), the / partition is only about 30 GB while my /home partition is about 300 GB, so it wouldn't even fit within /.
In the end, I just reinstalled. (Not much installed at that point anyway.) The installer program does allow for mounting old partitions without formatting, though it's a bit counterintuitive how to do so -- it required clicking the "mount" button, then clicking something else, then un-clicking "format". Kinda dangerous! It'd be better to have some clear method of mounting old partitions without automatically assuming the user wants to format them. Still, in the end, I got my old /home partition mounted as my new /home partition, and all is good. Firefox started right up in the same session I had back in Ubuntu 10.04!