Black rectangle rings true here. I get it pretty reliably on my phone (preview nightly, Android). If I click a GitHub link and FastHub automatically opens (any GitHub link), when I go back on the phone I see a black rectangle for the whole page.
That sounds like a separate issue. Similar to one I had before on android, but fixed. If you go to settings, about Firefox nightly, what version does it say?
Does the black go away by itself after a couple seconds, or when you scroll, or does it stay forever?
Pixel 3. Hopefully it's a common codepath.