Firejail is a low-level application that uses Linux kernel features to protect your system in certain adjustable ways. It ought to be used when running untrusted software in general.

The Linux version uses a shell script and a library named gog_com.shlib to start by default, which produces an error when launched with Firejail (even without any options). If the game was started directly by the Torchlight2 binaries (x86 and x86_64) it won't work with Firejail either.

Using the start.sh script will simply close the game right after launching (the launch screen will appear for a split second) with the error log pointing towards line 94 of start.sh. Using the Torchlight2.bin files directly will result in the same behaviour without any error pointers.

I'm not sure how to debug this further.

Starting the Linux version _without_ Firejail works though.
Starting the Windows version _with_ Firejail works, too. So I'm using the Windows version for the time being.