Users switching to a newer kernel should not have to install newer firmware files to keep their hardware working. At the same time updated firmware files must not cause any regressions for users of older kernel releases.
Drivers that use firmware from linux-firmware should follow the rules in this guide. (Where there is limited control of the firmware, i.e. company doesn't support Linux, firmwares sourced from misc places, then of course these rules will not apply strictly.)
Firmware files shall be designed in a way that it allows checking for firmware ABI version changes. It is recommended that firmware files be versioned with at least a major/minor version. It is suggested that the firmware files in linux-firmware be named with some device specific name, and just the major version. The firmware version should be stored in the firmware header, or as an exception, as part of the firmware file name, in order to let the driver detact any non-ABI fixes/changes. The firmware files in linux-firmware should be overwritten with the newest compatible major version. Newer major version firmware shall remain compatible with all kernels that load that major number.
If the kernel support for the hardware is normally inactive, or the hardware isn't available for public consumption, this can be ignored, until the first kernel release that enables that hardware. This means no major version bumps without the kernel retaining backwards compatibility for the older major versions. Minor version bumps should not introduce new features that newer kernels depend on non-optionally.
If a security fix needs lockstep firmware and kernel fixes in order to be successful, then all supported major versions in the linux-firmware repo that are required by currently supported stable/LTS kernels, should be updated with the security fix. The kernel patches should detect if the firmware is new enough to declare if the security issue is fixed. All communications around security fixes should point at both the firmware and kernel fixes. If a security fix requires deprecating old major versions, then this should only be done as a last option, and be stated clearly in all communications.