Skywriter - An extensible and interoperable web-based framework for code editing.
Treeherder - A detective tool that allows developers to manage software builds and to correlate build failures on various platforms and configurations with particular code changes (Predecessors: TBPL and Tinderbox).
Network Security Services (NSS) - A set of libraries designed to support cross-platform development of security-enabled client and server applications.
Network Security Services for Java (JSS) - A Java interface to NSS.
Personal Security Manager (PSM) - A set of libraries that performs cryptographic operations on behalf of a client application.
Other tools
Client Customization Kit (CCK) - A set of tools that helps distributors customize and distribute the client.
Mozilla Directory SDK - For writing applications which access, manage, and update the information stored in an LDAP directory.
Mozilla Raindrop - Was an upcoming technology for sending messages.
Mstone - A multi-protocol stress and performance measurement tool.
Thimble - Mozilla's web-based educational code editor, part of the company's "Webmakers" project (Thimble was shut down in December 2019 and its projects were migrated to Glitch[1]).