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 looks like

You’ll be successful in this role if you look beyond shipping features and improve the architecture, reliability, security, and long-term health of the product. You’re comfortable solving ambiguous problems, flagging risks early, documenting decisions, and collaborating across engineering, product, UX, users, and the open-source community.

Over time, your work will help Thunderbird’s iOS experience become more resilient, scalable, and easier for both staff and community contributors to build on. 

Work environment

This is a full-time, fully remote position. You’ll join a distributed team of Thunderbird staff, open-source community members, and contributors from around the world.

We rely on clear communication, thoughtful documentation, and collaborative decision-making to work effectively across time zones and disciplines.

Compensation & benefits

We benchmark our base salaries to local markets and target the 60th percentile of the peer market. The salary ranges for this role are:

• US: $123,000 - $144,000 USD

We may consider candidates with strong skills but less than the required experience. Title, level and compensation will be determined based on qualifications and experience.

In addition to competitive salaries, we offer a comprehensive benefits package designed to support your whole self.

Work & career

• Fully remote work & schedule flexibility
• Company-provided laptop
• Annual bonus program
• Monthly remote work stipend
• Annual professional development stipend
• Access to Coursera l earning platform
• Industry conferences
• Company all-hands and team gatherings

Rest & play

• 24 days PTO per year (prorated) 
• Your birthday
• Year-end company shutdown
• 9 wellbeing days 
• Public holidays
• Other paid leave
• Quarterly wellbeing stipend for personal / family activities

Health & family

• 401(k) co ntributions
• Health, dental, & vision insurance
• Disability insurance
• Life insurance
• Employee assistance program 
• Paid parental leave
• Paid sick days 

Work eligibility

Applicants must reside in and have permanent work authorization for the country location(s) specified in the posting. We are unable to consider applicants outside of these markets at this time. And, we do not provide visa sponsorship.

How to apply

Please apply directly through our career page . We carefully review every cover letter and screening question, so take the time to answer each fully. We value authentic, thoughtful responses that reflect your own experience and perspective. It is fine to use AI tools to polish your writing, but your answers should be your own. Candidates who submit generic or unoriginal AI-generated responses may be disqualified from further consideration.

Our commitment to diversity, equity, and inclusion

MZLA believes in the value of diverse creative practices and forms of knowledge, and knows diversity, equity and inclusion are crucial to and enrich the company’s core mission. We encourage applications from everyone, including members of all equity-seeking communities, such as (but not limited to) women, racialized and Indigenous persons, persons with disabilities, persons of all sexual orientations, gender identities and expressions.

We are an equal opportunity employer. We do not discriminate on the basis of race (including hairstyle and texture), religion (including religious grooming and dress practices), gender, gender identity, gender expression, color, national origin, pregnancy, ancestry, domestic partner status, disability, sexual orientation, age, genetic predisposition, medical condition, marital status, citizenship status, military or veteran status, or any other basis covered by applicable laws. MZLA will not tolerate discrimination or harassment based on any of these characteristics or any other unlawful behavior, conduct, or purpose. 

We will ensure that qualified individuals with disabilities are provided reasonable accommodations to participate in the job application or interview process, to perform essential job functions, and to receive other benefits and privileges of employment, as appropriate. Please contact us at View email address on us.fitly.work to request accommodation.