Testing Software

Thunderbird

sudo add-apt-repository ppa:mozillateam/thunderbird-next
sudo apt update; sudo apt upgrade

Firefox

sudo add-apt-repository ppa:mozillateam/firefox-next
sudo apt update; sudo apt upgrade

Gajim (nightly version)

sudo apt-get remove gajim
wget -q https://gajim.org/gajim.key -O - | sudo apt-key add -
sudo sh -c "echo deb ftp://ftp.gajim.org/debian unstable main > /etc/apt/sources.list.d/gajim.list"
sudo apt-get update
sudo apt-get install gajim-nightly

Rolling back

sudo rm /etc/apt/sources.list.d/gajim.list
sudo apt update
sudo apt remove gajim-nightly
sudo apt install gajim