On 6.9.0 RC1 if you attempt to adjust the disk temp thresholds values for the CACHE drive only, they will not save after hitting apply. All other cache and array drives update accordingly however:
Warning disk temperature threshold (°C)
Critical disk temperature threshold (°C)
I'm currently dumbfounded...
The issue seems to now be specific to FireFox (although I had never thought previously to test with a different browser prior to the 6.9.0 Stable release).
e.g., When updating the temp values in FireFox, they appear to save initially, but then revert randomly.
In Chrome, when updating the values, almost immediately, you can see a visual difference. The default values are denoted in a blue toned color rather than being a grayish color in FireFox.