Hello.

firmware-misc-nonfree provides firmware for Chelsio T6 NIC.  This
package also provides configuration files for the firmware.
Unfortunately the config files are ignored in practice, because the
driver is loaded from initramfs, and firmware is loaded at the same
stage.  Initramfs image does not contain configuration files.

I've fixed the issue on my machine with a following script:
cat /etc/initramfs-tools/hooks/ct6-config
#!/bin/sh -e

if [ "$1" = "prereqs" ]; then exit 0; fi
. /usr/share/initramfs-tools/hook-functions

cp -r /lib/firmware/cxgb4/configs/ $DESTDIR/lib/firmware/cxgb4/
cp -a /lib/firmware/cxgb4/*config.txt $DESTDIR/lib/firmware/cxgb4/

May be such a script can be included into firmware-misc-nonfree package.

Thank you very much for your work!