AOSP 7.1.2 - Firefox (Fennec and IceCat) result with just a white screen

I have installed a Custom ROM (AOSP 7.1.2) on a Samsung tablet. It is rooted, and also has gapps (pico). Most everything works - except, when I installed both versions of Firefox for mobile - Fennec and IceCat (both via F-Droid), and the installation was successful (though neither of them asked for any permissions). However, when I start either of them, they simply start as a white screen, and ... nothing happens (Fennec occasionally crashes after a few seconds of white screen, I have not seen IceCat crash so far, it just keeps at the white screen). I captured a bit of logcat - first fennec is started, then icecat - it has been pasted here: https://pastebin.com/ijGSr8X0 Would anyone have any idea as to why specifically these two Firefox apps start with nothing else but a white screen; and how to fix that? (BTW, I use the same IceCat on an Android 4.x device; it works fine there.)