Yes, kernel 6.12 is the most recent LTS, supported until at least December 2026.
As for linux-firmware, the firmware was split up into individual packages, and for the sake of updating, we created the linux-firmware-meta package, which is a meta-package that pulls in all other firmware packages.
When updating, you will be asked whether to replace linux-fimware with linux-firmware-meta. You must answer Y here.
Then, after updating, you can remove the individual firmware packages that you do not need, as well as linux-firmware-meta itself, but then you must also mark all of the firmware packages that you wish to keep as explicitly installed, like so… 
sudo pacman -D --asexplicit <name-of-package>
If you do not do this, then the firmware packages you wanted to keep will be counted as orphans.