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/
no. connecting it to the internet does not automatically mean using whatever websites that could infect you. be more cautious, limited web browsing and no public wifi, bluetooth off, and you will be fine.