HTML 5, JavaScript and CSS since 2015

Firefox OS, it's over

Mozilla confirmed that it abandons distribution and design of smartphones based on Firefox OS. This system was designed for cheap mobile phone to the third world market but this segment was instead filled by devices based on Android. Lack of applications and also the lack of responsiveness of the system on low-end devices had made it unattractive.
Mozilla is poised to reuse the system in the IoT.
December 9, 2015.

Update: Mozilla is definitely abandoning the idea of using the OS in the Internet of Thing, although a product has successfully used it, a Panasonic TV (and maybe because there is only one product) and fired 50 people.
3 February 2017.

Why Mozilla drops Thunderbird

The development of the email client, considered by many as the current best, was left to contributors since 2012. Now Mozilla wants to completely abandon it and not host it at all, and seeks for an independant host.
The reason seems specious: they want to devote to leading products, namely Firefox. Thunderbird which is widely used is not considered a significant product. This is part of the contradictions of the group, which recently announced that it did not need money from Google, its revenue being sufficient, before launching a campaign for donations from users of Firefox!
This comes at a technical level in the scope of the abandonment of XUL in favor of HTML 5: Thunderbird is a software based on XUL, like Firefox, but Mozilla looks to HTML 5 for the future, especially with the Servo browser currently in development.
December 2, 2015.

The HTML browser is here... but comes from Microsoft!

It is ultimately not Mozilla which will replace Xul by HTML, for the moment, but Microsoft which offers a browser written in JavaScript, HTML and CSS and it is open source, available on GitHub under the name JSBrowser.
The rendering engine is written in native code, it is EdgeHTML used by the Edge browser.
August 29, 2015.

New architecture for Firefox in project

After the Firefox experiment with HTML 5 interface, Mozilla actually is thinking about changing its interface language. In a note of July 6, the abandonment of XUL and XBL is clearly mentioned. The goal is to build Firefox only with Web technologies, which have more resources because of their wide implementation, unlike XUL. Currently we do not even know what interface will replace it in Gecko. HTML 5 or native widgets like on Android? It will require a lot of time before this transition happen.
Because one does not even evokes Servo, the new browser under development, there are chances that it never exists as such: it will rather be the new Firefox rewritten in another programming language. But that's for a future even more distant.
July 7, 2015.