It's always been the same firmware.  firmware is firmware.  Different distros
have changed their packaging polices with respect to firmware.  That might be
the case for you (e.g., distro used to just include the firmware with the
kernel and now ships it as a separate package).  As to narrowing it down, I'd
suggest trying interim kernels between the last know good one and most recent
one you've tried to see of you can narrow down when it broke.  E.g., if kernel
3.14 worked and 3.15 is bad you know the regression happened in the 3.15 kernel
cycle.