If you are in a situation when you want to upgrade your system, but do not want to upgrade a certain software in it you can just instruct apt not to upgrade these packages with:
apt-mark hold <package name(s)>
Here is how you can block updating 4 packages – ca-certificates, firefox, ghostscript, linux-firmware. First we update and upgrade and you can see there is no packages to keep back, and then we use apt-mark to “hold” package “linux-firmware” and ca-certificates, firefox, ghostscript at once. Initiating apt upgrade again will give you “The following packages have been kept back:” and it will include all packages, which will not be upgraded (it will include dependencies, which require some of the blocked packages).
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