Looks like Fenix dropped support for Android 5/6/7

I have quite a few older Android devices, my main device runs 8(O) and I have some on 7(N), 6(M), 5(L). 4.1~4.4 support was already gone with Fenix, but now it seems like they got 5/6/7 as well. https://f-droid.org/en/packages/org.mozilla.fennec_fdroid/