Idempotent: install Firefox default profile if none has been set on this thread.
Called by globals::install_all so fetch() gets TLS/HTTP2 fingerprints by default.
Clone the current thread’s stealth profile. Single source shared by every
page egress path (fetch, WebSocket wss://) so all TLS handshakes from one
page present the identical JA3/JA4 fingerprint (REQ-STL-001 fingerprint
consistency).