Manjaro previously had linux-firmware package overlays to support older kernels. That is no longer necessary, so they were dropped in favor of the original Arch packages.
linux-firmware is just a meta package to pull in all the firmware packages, so it can either be installed manually to replace linux-firmware-meta or the latter can be removed.
To mark the package(s) you want to keep as explicitly installed, use the Mark as explicitly installed button in Add/Remove Software or via CLI:
sudo pacman -D --asexplicit <package-name1> <package-name2>