According to Debian rules for free software, Firefox and Thunderbird aren't supplied with Debian. In place, Debian supplied Iceweasel and Icedob, which are rebranded versions of Firefox and Thunderbird without logos and copyrighted names. Here is a way to retrieve our favorits softwares on our favorit but-a-little-punctilious linux flavor.
We will create a new apt source which reference a repository maintained by ubuntuzillateam. Don't be afraid, this is not an Ubuntu-only project, just a project born in Ubuntu Forum.
First, create a new file /etc/apt/sources.list.d/ubuntuzilla.list, like this (as root) :
echo 'deb http://downloads.sourceforge.net/project/ubuntuzilla/mozilla/apt all main' > /etc/apt/sources.list.d/ubuntuzilla.list
Next, add repository security key, update and remove Debian official packages:
apt-key adv --recv-keys --keyserver keyserver.ubuntu.com C1289A29
apt remove iceweasel icedove
Finally, install firefox and thunderbird :
apt install firefox thunderbird