KaiOS is a mobile operating system based on Linux, developed by KaiOS Technologies (Hong Kong) Limited, a company based in Kowloon, Hong Kong, with largest shareholder being Chinese multinational electronics conglomerate TCL Corporation. It is forked from B2G OS (Boot to Gecko OS), an open source community-driven fork of Firefox OS, which was discontinued by Mozilla in 2016.

The primary features of KaiOS bring support for 4G LTE E, VoLTE, GPS and Wi-Fi with HTML5-based apps and longer battery life to non-touch devices with optimized user interface, less memory and energy consumption. It also features over-the-air updates. A dedicated app marketplace (KaiStore) enables users to download applications. Some services are preloaded as HTML5 applications, including Twitter, Facebook and YouTube. As of April 1st, 2020 there are 500+ apps in KaiStore. The operating system is comparatively lightweight on hardware resource usage, and is able to run on devices with just 256 MB of memory.

The operating system first appeared in 2017 and is developed by KaiOS Technologies Inc., a Kowloon, Hong Kong-based company headed by CEO Sebastien Codeville with offices in other countries. In June 2018, Google invested US$22 million in the operating system. India-based telecom operator Reliance Jio also invested $7 million in cash for a 16% stake in the company.

In market share study results announced in May 2018, KaiOS beat Apple's iOS for second place in India, while Android dominates with 71%, albeit down by 9%. KaiOS growth is being largely attributed to popularity of the competitively-priced Jio Phone. In Q1 2018, 23 million KaiOS devices were shipped.

In March 2020 Mozilla and KaiOS Technologies announced a partnership to update KaiOS with a modern version of the Gecko browser engine and more closely aligned testing infrastructure. This change should give KaiOS 4 years worth of performance and security improvements and new features, including TLS 1.3, WebAssembly, WebGL 2.0, Progressive Web Apps, new video codecs like WebP, AV1 and modern JavaScript and CSS features.

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