I've had to research this not having a linux mint box at present. This is what I saw as the unmodified sources.list file which is placed in /etc/apt/sources.list.d/official-package-repositories.list in linux mint:
Code:
deb http://packages.linuxmint.com elsie main upstream import backport #id:linuxmint_main
deb https://deb.debian.org/debian bullseye main contrib non-free
deb https://deb.debian.org/debian bullseye-updates main contrib non-free
deb http://security.debian.org bullseye-security main contrib non-free
deb https://deb.debian.org/debian bullseye-backports main contrib non-free
It is evident that it includes the non-free repositories, and it is certain that the package firmware-misc-nonfree is available in that debian repository set in this sources list above.
If you could compare your sources list in /etc/apt/sources.list, and the sources in /etc/apt/sources.list.d/official-package-repositories.list with the above and see what is missing, that would help.
If you have the line:
Code:
deb https://deb.debian.org/debian bullseye main contrib non-free
in your sources list, then the firmware-misc-nonfree package will be available to be installed by apt-get.
If by chance that repository is already there on your system after your attempts to install it and failed to, then it's a conundrum that I can't get at the moment.
If that repository doesn't appear, you could include it, and then run apt-get again, update and install the firmware package.
An alternative to the above approach is to download the package from a debian or linux mint repository
and install it with the dpkg command.
First however, install the package: initramfs-tools, if it is not already installed. Use apt-get to install that because it's recommended for the firmware package. Then, download the firmware package and install it with dpkg with a command like:
Code:
dpkg -i firmware-misc-nonfree_20210315-3_all.deb
The package is available in the directory here:
http://ftp.au.debian.org/debian/pool/non-free/f/firmware-nonfree/