Senior Mobile Engineer, iOS

Why Thunderbird? Thunderbird is one of the world s most trusted open-source email applications, empowering more than 20 million people globally. At MZLA, the team behind Thunderbird, we build privacy-respecting communication and productivity tools that help people manage their digital lives while staying in control of their data. We are open source by design. Our products are developed transparently and in collaboration with a global community of contributors, and that same spirit shapes how we work: collaboratively, inclusively, and with a shared commitment to putting users first. We are a small but growing company of 60+ people distributed across seven countries. As Thunderbird evolves beyond a single desktop application, we are expanding across desktop, mobile, paid services, and new product experiences that help people and organizations work more effectively without giving up control to big tech ecosystems. Our revenue model combines user donations, which help keep Thunderbird s apps freely available, with paid services that cover the costs of hosted offerings while supporting long-term sustainability and continued innovation. A note about MZLA and Mozilla: MZLA Technologies Corporation is the nonprofit-owned company behind Thunderbird and part of the Mozilla family. MZLA is separate from Mozilla Corporation and has its own compensation programs, benefits, and employment policies. To learn more, visit The opportunity The Senior Software Engineer, iOS is an experienced individual contributor who brings deep iOS expertise to the design, development, and evolution of Thunderbird s iOS application. This role will continue to contribute to Thunderbirds effort to develop the iOS app and establish a strong technical foundation for the product. You ll work closely with mobile engineers, product management, UX, and other partners to deliver a high-quality iOS experience that meets high standards for performance, reliability, accessibility, security, and maintainability. You ll play an important role in delivering exceptional user experiences for Thunderbird s global user base by translating product needs into thoughtful technical solutions and ensuring your work aligns with MZLA s mission and the objectives of the Thunderbird Project. You ll also collaborate with open-source contributors and mobile engineers across the broader Mozilla ecosystem. What you ll do • Build and refine Thunderbird s mobile products and platform architecture. • Improve scalability, performance, reliability, and user experience. • Develop a deep understanding of the codebase and architecture to confidently make changes across the entire stack. • Maintain strong debugging and testing practices across unit, integration, and automated testing. • Review code and guide improvements to architecture, testing, and development best practices. • Independently resolve complex issues involving performance, security, and cross-platform integration. • Monitor application health and address issues before they escalate. • Document technical decisions, system architecture, and team processes. • Own assigned projects, collaborating cross-functionally, from investigation and planning through development, validation, and delivery. • Ensure mobile development meets accessibility, security, and privacy standards. What you bring • 7+ years of professional software development experience, including 2+ years in a Senior Software Engineer role. • 3+ years of professional mobile application development experience. • Strong experience with Swift, SwiftUI, and iOS frameworks. • Deep knowledge of iOS architecture, design patterns, and performance optimization. • Experience delivering and maintaining complex, scalable, and reliable software. • Experience addressing technical debt and refactoring large or complex codebases. • Strong knowledge of mobile security, including encryption, secure storage, app signing, and vulnerability remediation. • Experience with communication and email protocols such as RSS, IMAP, POP, or Matrix, and familiarity with the IETF standards for CalDav or CardDav. • Advanced proficiency with Git and collaborative development workflows. • Strong debugging, testing, and problem-solving skills. • Strong attention to detail, curiosity, and a product-focused mindset. • The ability to work independently, take ownership, and collaborate across teams. • Respect for diverse skills, experiences, and perspectives. • Excellent async written communication skills; comfortable working with a geographically distributed team. • Ability to learn, evaluate, and responsibly use emerging technologies, including AI-enabled tools, to improve work processes. Bonus points for • Familiarity with Apple App Store and Google Play Store compliance guidelines. • Experience contributing to open-source projects or communities. • Proficiency in French, German, Japanese, or another widely supported Thunderbird language. What success loo...