I just started testing v6. Going into Settings > DNS > Custom DNS servers, deselecting the two that were selected in the list, pasting in a couple of lines for custom external DNS servers and clicking Save & Apply. Page reloads and the (now all deselected) DNS servers list fails to load, just shows the spinner endlessly.
Manually reloading the page renders it correctly and shows it all worked okay.
*** No support needed, just feeding back in case useful.
Core
Version is v5.17.1-253-g719ce801 (Latest: v5.17.2)
Branch is development-v6
Hash is 719ce801 (Latest: 719ce801)
Web
Version is v5.19-592-ga0abbb5a (Latest: v5.21)
Branch is development-v6
Hash is a0abbb5a (Latest: a0abbb5a)
FTL
Version is vDev-d6f13ac (Latest: v5.23)
Branch is development-v6
Hash is d6f13ac7 (Latest: d6f13ac7)
Firefox 120.0.1 (64-bit) on Mint Linux