xserver-xorg-video-cirrus_1.5.3-1+b3_amd64.deb for Debian Stable (Bullseye) does seem to be at
deb http://ftp.de.debian.org/debian bullseye main
but that is not a backport.
When I look at https://packages.debian.org and search "bullseye-updates" and "bullseye-backports"
I get this:
"You have searched for packages that names contain cirrus in suite(s) bullseye-backports, all sections, and all architectures.
Sorry, your search gave no results".
Searching "bookworm" (testing) does show:
xserver-xorg-video-cirrus_1.5.3-1+b5_amd64.deb
but I don't know if I can use that with my Debian 11.6 "live" usb stick.
I hate to sound stupid, but could someone please tell me how to add that to the
Debian 11.6 "live" usb stick?
I know how to enable the bullseye backports repository in
/etc/apt/sources.list, but then I dont know how to "get to" and use backports; I've never had to do that before.