Mozilla has decided to drop Firefox OS for smartphones. According to xda-developers, this notification was sent out to developers via email. Also, a copy of this email is shared on Mozilla Discourse page for Firefox users to be aware of the new development.
We will end development on Firefox OS for smartphones after the version 2.6 release. This means that Firefox OS for smartphones will no longer have staff involvement beyond May, – Mozilla
The decision to shutdown the OS is because of the fact that it could not compete with other matured operating systems which are already deep-rooted and Mozilla does not foresee any success on commercial smartphones.