--5.9 was a new major branch of firmware for 5th generation and 5.5 generation devices. --6.1 was developed for 6th generation devices, but was based off of 5.8.
For reference, below are examples of 5th generation, 5.5 generation, and 6th generation devices:
6.2 was later developed for 6th generation devices, and this was based off of 5.9. So while either 5.9 or 6.1 based settings can be imported into 6.2 firmware, 5.9 settings cannot be imported into 6.1 firmware, as this would be equivalent to cramming 5.9 based settings into an engine based off of 5.8 firmware (6.1).
NOTE: 6.2 firmware has since been provided as a general release.