Is Firefox Nightly a good browser to use for security and privacy reasons? Because new security and crypto technologies are implemented faster in FF Nightly, but what about 0 Day security issues? Should I switch from stable to nightly?
IMO, it's a trade-off only you can assess. As you said, it's probable that Nightly is ahead of stable regarding new security-related technologies. However, those new features are not as tested as the features in stable, so actually they might cause more harm than benefit.
Would you risk a stable browser for a less-tested more error-prone browser just to use the latest technologies? It's up to you.