Debian : $ sudo apt -y install firejail firetools
Arch : $ sudo pacman -S firejail firetools
After that is done, start the Firetools app, shut down your running Firefox instance, and then fire it up from the Firetools app. To check if it does its job, type in the Firefox address bar :
Then browse into your home directory. If all is well it should only list your Downloads directory.
Happy browsing ! :)
Two interesting articles on Firejail :
Image source : "Source code security plugin" by Christiaan Colen is licensed under CC BY-SA 2.0