If you do have the right firmware then you'll be able to configure your connection with the network-manager when you first log in GNOME. Personally I never configure wifi during install, in part because I've had problems like yours. The machine I'm using right now needs firmware-iwlwifi-4965, which I put there after the install, without ethernet.
What I might do in your situation - free about 15 Gb on your Ubuntu partition, install Debian there, and then if you're happy and all is working well, perhaps just create a new partition with the Ubuntu space and mount it as /home.
Hard work pays off in the future. Laziness pays off now.