According to the Mozilla blog:
At Mozilla, our mission is to bring the Web to as many people as we can. Given that roughly half of the nearly 500 million Android phones in use today run on ARMv6 architecture, this is an important step toward making the open Web free to all.
An important new accessibility feature in this version is integration with TalkBack, the Android screen reader, with no need for additional configuration or installations. This makes browsing the Web on a mobile phone easier for the visually impaired.
It also supports JellyBean's Explore by Touch, a feature that makes it easier for the visually impaired to browse the Web using speech output, sounds and vibrations, and Gesture Navigation.
For developers this release provides an incentive not only to develop for Firefox on mobile devices but also for Firefox OS. Firefox is now in an even better position to be a dominant force in the mobile market.