Installation:

sudo apt install mono-complete
sudo apt install keepass2

Integration in Firefox:

KeyFox

sudo cp $HOME/.mozilla/firefox/*.default/extensions/keefox@chris.tomlinson/deps/KeePassRPC.plgx /usr/lib/keepass2/plugins/

Tray-Icon für XFCE:

sudo apt-add-repository ppa:dlech/keepass2-plugins
sudo apt update
sudo apt install keepass2-plugin-application-indicator