Fire OS 5 mount -o remount,rw fails as root
You can also try with: mount -o rw,remount /system . For some devices this works fine.

root@angler:/ # mount -o rw,remount /system
root@angler:/ #
root@angler:/ #
root@angler:/ #
root@angler:/ #
root@angler:/ #
root@angler:/ #
root@angler:/ #
root@angler:/ #
root@angler:/ #
root@angler:/ # mount -o remount, rw /system
root@angler:/ #
root@angler:/ #
root@angler:/ #
root@angler:/ #
root@angler:/ # mount -o rw,remount /system
root@angler:/ #
root@angler:/ # getprop | grep build
[ro.bootimage.build.date]: [Fri May 6 06:19:55 EDT 2016]
[ro.bootimage.build.date.utc]: [1462529995]
[ro.bootimage.build.fingerprint]: [google/nexus_angler/angler:6.0.1/MTC19T/nate05060618:userdebug/test-keys]
[ro.build.characteristics]: [nosdcard]
[ro.build.date]: [Fri May  6 06:18:32 EDT 2016]
[ro.build.date.utc]: [1462529912]
[ro.build.description]: [angler-user 6.0.1 MTC19T 2741993 release-keys]
[ro.build.display.id]: [MTC19T]


I just did the test now on my device, Nexus 6P. As you see, both commands are valid and system applies them. Why was my answer down voted?