I use both on iOS. I generally open in Focus because it actually tends to be stricter. I find that if I open something in Focus I can, from there open in Firefox if blocking is too strict and sometimes it works. Another plus to using Focus, if a site is completely broken, I can just disable all blocking and not worry much about it because nothing gets persisted when I leave and blocking is automatically re-enabled the next time around.

Same workflow is pretty much possible without it, but it does make ephemeral browsing a bit easier.