[Bug-gnuzilla] Android - cannot configure Add-ons - denied permission WRITE_SETTINGS

[cotw]

Hello,

In Android 7.1.2 (MIUI 9.5.5.0) I installed Icecat 52.6.0 from F-Droid.org.

I cannot enter "preferences" of e.g. LibreJS addon, and also not of new add-ons 
I install, e.g. uBlock Origin.

Only if I "deactivate" e.g. Gnu LibreJS addon and then "activate" again, 
"preferences" button is visible for a short time. Even if I click it and change 
something, it seems, IceCat cannot keep the settings.

I never managed even betting dsiplayed uBlock Origin settings, or 
preferences/settings button under "addons", though.

Two permissions are shown "denied permissiona" in Android permissions:
- WRITE_SETTINGS
- SYSTEM_ALERT_WINDOW

I have tried all I could to grant any permssion. Also with root terminal:
pm grant org.gnu.icecat android.permission.WRITE_SETTINGS

That results in following error:
"Operation not allowed: java.lang.SecurityException: Permission 
android.permission.WRITE_SETTINGS is not a changeable permission type."

Is this my problem, Xiaomi MIUI 9.5.50 problem, Android 7.1.2 problem or 
IceCatMobile 52.6.0 problem?

Has anyone noticed the same misfunction or is this a known issue?

Thank you, cotw