So, for my "untrusted" qube, I decided I wanted Debian, and I changed it in the Qubes settings. That worked fine. However, when I did that, Firefox stopped coming up when I clicked on untrusted:Firefox (though it came up fine from the terminal when I typed "firefox").
So... I went back to the Qubes Settings -> Applications and only Files and Terminal were in the "Selected" column, though "Firefox ESR" was in the "Available" column. I moved Firefox ESR into the Selected column and it started coming up.
So, I gather, Firefox and Firefox extended release are not the same, and if you switch to the Debian template, you will need to change that...
billo