Firefox Notes allowed users to write and sync encrypted notes using a Firefox extension or an Android app. It will be decomissioned in November 2020, and notes will be available only for export.
Firefox Send was an end-to-end encrypted file sharing service. It was suspended in July 2020 due to abuse, and fully shut down in September 2020.
Firefox Hello was a video chat service built into Firefox versions 34 - 49. It allowed users to initiate video calls with one another by simply sharing a link, no account required. While Firefox Hello had good integration with Firefox, it could be used from any browser that supported WebRTC.
X-Ray Goggles was a browser extension that allowed users to easily view and edit the source code of any webpage, then share their modifications with others. It was decomissioned in December 2019.
Lightbeam was a Firefox extension, originally named Collusion, that recorded tracking cookies saved to the users' browser as they visited websites. It then visualized these cookies in a graph, showing when a cookie was used to track the user across websites. It was discontinued in 2019 in favour of Firefox’s Enhanced Tracking Protection, which unfortunately doesn’t provide the same level of visualization.
Firefox OS was an open-source operating system for smartphones, tablets, and smart TVs, based on Gecko (Firefox’s rendering engine), and the Linux Kernel. The sale of Firefox OS smartphones ended in 2015, and development was discontinued in 2016.
Appmaker was a tool to build mobile apps without code. Instead, it used visual blocks and signals. Appmaker, along with Popcorn Maker, was shut down in September 2015 to focus on Webmaker.
Thimble was an educational, web-based code editor that allowed users to easily create and publish websites. Thimble was shut down by December 2019, and users were given the chance to migrate their projects to Glitch, a similar tool for building web apps.
Firefox Tilt, which started as a browser extension, and later was added to the built-in developer tools, let you view web pages in 3D. Nested HTML elements would pop out of the page, and you could pan, rotate, and zoom to look around. A demo can be seen in this developer tools update video.
Mozilla Persona was a cross-browser, decentralized, single sign-in web authentication system. The project was shut down in 2016 due to low adoption.
Ubiquity was an experiment to add a natural-language-based command line to Firefox that helped users get tasks on the web done faster. Ubiquity commands were small pieces of code that any website could provide, which users could choose to subscribe to when available.
Deuxdrop was a distributed, secure messaging system, intended for one-to-one and group messaging. Deuxdrop was a continuation of Raindrop, a previous project with similar goals.
Mozilla Prism allowed users to run web apps directly on their desktop, outside of their web browser. It used Firefox under the hood, and added additional capabilities such as offline data storage and hardware accelerated graphics. Prism was available as both a standalone application and as a Firefox extension.
XULRunner allowed developers to create standalone desktop apps based on Gecko, using Mozilla’s XUL (XML User Interface Language). Both Firefox and Thunderbird were built on XULRunner, as well as several non-Mozilla products including Flickr Uploadr, Google Adwords Editor, IBM Lotus Notes, and more.
Mozilla Sunbird was a standalone, cross-platform calendar application. Development was discontinued in 2010 to focus on development of Lightning, which shared the same codebase, but was built into Mozilla Thunderbird.
Minimo was a version of Mozilla optimized for small devices, including cellphones and PDAs, that had limited resources. It was available for platforms including Windows Mobile, Windows CE, and GPE. It was later superseded by Firefox Mobile (code-named Fennec).
Camino was a web browser that was built for and integrated heavily with Mac OS X, based on Firefox’s Gecko engine. Camino had a native interface, as opposed to the XUL UI framework used by Firefox. The end of development was announced in 2013.
Vixen was supposed to be a visual IDE for Mozilla’s XUL UI. Its features would include a visual form designer that would allow developers to design windows and dialogs without writing code, similar to Delphi or Visual Basic. It seems like the project never got very far.
Grendel was a mail and news client written in Java, and built on Sun’s JavaMail API. It was an open-sourced portion of the code from Netscape Navigator’s Java rewrite, code-named Xena.
ElectricalFire was an open-source, just-in-time Java virtual machine. It was open-sourced by Netscape in 1999 after it was no longer being developed as a commercial product.
Mariner aimed to improve the performance and stability of Netscape Communicator. It was open-sourced in March of 1998, but was abandoned in October that year, as Netscape decided to work on its newer NGLayout (now known as Gecko) engine.
MXR was Mozilla’s fork of LXR, a source code indexer that aimed to improve code comprehension. Eventually Mozilla began working on a replacement tool with better static analysis and UI. This replacement is called DXR, and once it reached maturity, Mozilla shut down their MXR instance.