Just to warn you, this post is a bit bias , I personally prefer firefox, so my answer will obviously be firefox, yes, firefox is not the king of privacy, but I still think it's great, here's why.
Firefox is less heavy and less bloated, it's not as much of a resource, space and in general firefox is more light and is smaller than chrome, chromium, etc. it also takes less time to compile which matters to me a lot because I compile everything from source, chromium takes ~60 times as much to compile as firefox and chromium is written in C++ unlike firefox - in rust and rust is known to be slow at compiling so if firefox switched back to C++ - firefox would be even faster to compile.
Chromium also has a messier codebase so that also makes it prune to bugs and generally misshaps and stuff. Firefox's LoC is 21 milion where chromium has 35.
Firefox uses a less bloated, but less performant web engine - gecko, the efficiency is not bad imo, but slower than blink, though as it's lighter - i prefer it.
Firefox is easier to harden making it nice for people who are willing to spend the time tinkering around with it nicer, chromium is a bit a pain in the ass and as far as I see the most you can harden it easily is to install a few googled addons and that's it where with firefox you can do a lot more. Firefox is great for tinkerers and people who like customisation.
Firefox allows for more customisation through
script (mainly for preferences) and userChrome.css which is great again,
for people who are willing to spend the time customising it. I
prefonally made firefox's interface minimalist with my userChrome and
it's nice, where with chromium I don't think I can do that AFAIK.
Chromium and chrome are the most popular browsers out there, so they expect most people using them to be "tech normies" (explained what they are in one of my blog posts), so they add all that various abstraction and hiding making it hard for anyone to debug it.