HTTP cookies gave web applications a small client-side mechanism for carrying state across otherwise separate requests, enabling shopping carts and login sessions while also opening the door to cross-site tracking.
Netscape's SSL and the IETF's TLS turned encryption, authentication, and integrity into a reusable transport layer for web traffic, creating the technical basis for HTTPS and secure network applications.
Netscape's 1998 decision to release Communicator source code created the Mozilla project, turning a proprietary browser codebase into an open community and influencing the newly coined 'open source' movement.
JavaScript transformed the browser from a document viewer into a programmable environment, then survived browser competition by becoming an independently standardized language.