Workaround (ADB pre-enabled, no root required)
From your computer, launch on a terminal with your FP2 connected by USB:
adb shell settings put system screen_brightness_mode 0
adb shell settings put system screen_brightness 50
First command disables auto brightness. Second command may not be needed, but ensures brightness level is not low (range goes from 0 to 255).
For future reference,
settings’s keys can be found in Java constants on the Settings app source code (values in quotes after lines starting withpublic static final String)
PD: I know, probably few people had ADB enabled before the blackscreen… but there’s hope for that few people, 