$ sudo dmesg | grep 'firmware load for' | grep 'failed with error'
$ sudo dmesg | grep 'firmware: failed to load'

This will search your diagnostic kernel logs for failed firmware loads and will produce lines that look similar to the following

[   53.168802] bluetooth hci0: Direct firmware load for qca/rampatch_usb_00000200.bin failed with error -2

Take that file name and pass it to

$ sudo apt-file search qca/rampatch_usb_00000200.bin

Which in this case will yield

firmware-atheros: /lib/firmware/qca/rampatch_usb_00000200.bin

Finally, run

$ sudo apt install firmware-atheros

and reboot!