In this release, Iceweasel and Icedove have been replaced with Firefox and Thunderbird now that the project is allowed to include those trademarks in its releases. MariaDB has been made the default MySQL variant, this switch will happen automatically for anyone upgrading to the new release.
With regards to security, this is a major release. A new version of GnuPG has been included which brings support for elliptic curve cryptography, better default options, a more modular architecture, and improved smartcard support. The X display system no longer requires root privileges to run, and now protections are in place to protect users from malicious attempts to tamper with compilers and build networks; 90% of source package in Debian 9 will build bit-for-bit binary packages for verification purposes.
Debian 9 comes with lots of updated packages. The Linux Kernel is now on version 4.9 and LibreOffice is on version 5.2. All of the major desktop environments have been updated to more modern versions too. You can find a more comprehensive list of the software that was updated in the announcement.
Lastly, support has been added for the mips64el architecture for 64-bit little-endian hardware, but support for 32-bit Motorola/IBM PowerPC (powerpc) has been removed.
Source | Download Debian 9 Stretch