/lib/udev/rules.d/50-firmware.rules is in udev 229-4ubuntu4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | # stub for immediately telling the kernel that userspace firmware loading
# failed; necessary to avoid long timeouts with CONFIG_FW_LOADER_USER_HELPER=y
SUBSYSTEM=="firmware", ACTION=="add", ATTR{loading}="-1"
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