www.mozilla.org/en-US/firefox/android/39.0/releasenotes/

What’s New
NEW
Paste Android clipboard content into editable web content
NEW
Support for 'switch' role in ARIA 1.1 (web accessibility)
CHANGED
Removed support for insecure SSLv3 for network communications
CHANGED
Disable use of RC4 except for temporarily whitelisted hosts
CHANGED
Sync now only uses the strongest TLS configuration available on your device. If you self-host Sync, see docs.services.mozilla.com/howtos/configure-tls.html for details.
HTML5
Enable the Fetch API by default
HTML5
List-style-type now accepts a string value
HTML5
Implement
HTML5
Cascading of CSS transitions and animations now matches the current spec
HTML5
Added support for CSS Scroll Snap Points
FIXED
Improve performance for IPv6 fallback to IPv4
FIXED
Fix incomplete downloads being marked as complete by detecting broken HTTP1.1 transfers
FIXED
Various security fixes

This release does not have a Play Store Description, so we grabbed one from version 55.0.2:

Experience a fast, smart and personal Web. Firefox is the independent, people-first browser made by Mozilla, voted the Most Trusted Internet Company for Privacy. Upgrade today and join hundreds of millions who depend on Firefox for a more personal browsing experience.

FAST. SMART. YOURS.
Firefox is made with you in mind and gives you the power to take back control of your Web experience.
That’s why we design the product with smart features that take the guesswork out of browsing.

SEARCH INTELLIGENTLY & GET THERE FASTER
- Firefox anticipates your needs and intuitively provides multiple suggested and previously searched results across your favorite search engines. Every time.
- Easily access shortcuts to search providers including Wikipedia, Twitter and Amazon.

NEXT LEVEL PRIVACY
- Your privacy has been upgraded. Private Browsing with Tracking Protection blocks parts of Web pages that may track your browsing activity.

SYNC FIREFOX ACROSS YOUR DEVICES
- With a Firefox Account, access your history, bookmarks and open tabs from your desktop on your smartphone and tablet.
- Firefox also safely remembers your passwords across devices so you don’t have to.

INTUITIVE VISUAL TABS
- Intuitive visual and numbered tabs easily let you find content for future reference.
- Open as many tabs as you like without losing track of your open Web pages.

EASY ACCESS TO YOUR TOP SITES
- Spend your time reading your favorites sites instead of looking for them.

ADD-ONS FOR EVERYTHING
- Take control of your Web experience by personalizing Firefox with add-ons like ad blockers, password and download managers and more.

QUICK SHARE
- Firefox remembers your most recently used apps to help you easily share content to Facebook, Twitter, WhatsApp, Skype and more.

TAKE IT TO THE BIG SCREEN
- Send video and Web content from your smartphone or tablet to any TV equipped with supported streaming capabilities.

Learn more about Firefox for Android:
- Have questions or need help? Visit support.mozilla.org/mobile
- Read about Firefox permissions: mzl.la/Permissions
- Learn more about what’s up at Mozilla: blog.mozilla.org
- Like Firefox on Facebook: mzl.la/FXFacebook
- Follow Firefox on Twitter: mzl.la/FXTwitter

ABOUT MOZILLA
Mozilla exists to build the Internet as a public resource accessible to all because we believe open and free is better than closed and controlled. We build products like Firefox to promote choice and transparency and give people more control over their lives online. Learn more at www.mozilla.org

Privacy Policy: www.mozilla.org/legal/privacy/firefox.html