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# Senior Software Engineer, WebExtensions

- **Company:** Mozilla
- **Location:** UK (Remote available)
- **Experience:** Expert
- **Salary:** £66,000.0 - £87,000.0
- **Contract:** Permanent contract
- **Skills:** HTML, JavaScript (Programming Language), Application Programming Interfaces (APIs), Browser Compatibility, Bugzilla, C++ (Programming Language), Cascading Style Sheets (CSS), Code Review, Software Debugging, Firefox, Gecko (Software), Github, Design of User Interfaces, Open Source Technology, Web Application Security, Software Engineering, Free and Open-Source Software, Api Design
- **Published:** May 15, 2026
- **Apply:** https://uk.indeed.com/viewjob?jk=a09db1a7ad89c0ea

## About the Role

Do you have experience in Software development?, * 4+ years of professional experience in software development.

 * Strong proficiency in JavaScript, DOM APIs, HTML, and CSS.
 * Familiarity with modern browser architecture and debugging tools.
 * Experience with API design and integration.
 * Ability to navigate and learn from large codebases.
 * Strong communication skills and experience collaborating in distributed teams.
Bonus points for:
*

 + Experience with Firefox internals, C++ or Rust.
 + Familiarity with Android or GeckoView development.
 + Knowledge of web security principles.
 + Contributions to open source projects, especially in browser or extension development.

## Description

The WebExtensions platform powers the add-ons ecosystem in Firefox, enabling millions of users to customize their browser while keeping safety and performance at the forefront. As a WebExtensions Engineer, you will help implement new extension capabilities, improve existing APIs, and ensure that extensions remain reliable across Firefox.

As a Software Engineer on the WebExtensions team, you'll bring a passion for exploring complex systems and creating experiences that balance developer power with user trust. You'll work deep in the Gecko platform to evolve extension capabilities, and you'll build UI features in Firefox that make extensions clear and manageable for users. Along the way, you'll collaborate with experts across Mozilla and help advance cross-browser compatibility and the path toward standardization. Your work will keep extensions safe, powerful, and reliable for developers and millions of Firefox users.

What you'll do:

 * Implement and maintain WebExtensions APIs and Addon Management, balancing power, safety, and performance, as well as user interfaces in Firefox.
 * Debug and fix issues affecting extensions in Firefox.
 * Collaborate with product managers, QA, and the AMO engineering team to deliver features end-to-end.
 * Contribute to Mozilla-owned extensions and system add-ons.
 * Participate in open source workflows: code reviews, pull requests, and discussions in Bugzilla/GitHub.
 * Support extension developers by helping diagnose issues and improving tooling/documentation.


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