Settings cheatsheet: Tor Browser is up to date (about: version reports the current stable release); the security slider in the shield icon is on Safest; the search default is set to whatever you prefer (does not matter for Nexus access); HTTPS-Only mode is on (does not affect onion services but useful for clearnet hops); the New Tor Circuit menu item is reachable from the toolbar circuit-icon dropdown.
That's it. There is no Nexus-specific Tor Browser configuration; the platform is reachable on a default install with the slider correctly set. The verification flow happens after the page loads, not in the browser config.
Verified working Nexus Market mirrors
Three v3 onion addresses currently serving the production market, signed under PGP fingerprint 0x7F2A0A9D. Use the Copy button; never retype an address.