Ubuntu or Debian:

$ sudo apt-get update

$ sudo apt-get install firejail

$ sudo apt-get install xpra xserver-xephyr

The sandbox replaces the regular X11 server with Xpra or Xephyr server. This prevents X11 keyboard loggers and screenshot utilities from accessing the main X11 server.

To install for all, Users (NOTE: you'll need to make the "work" folder for each users home path on the PC):
IE: $ mkdir /home/USERsName/work
$  mv /usr/share/applications/firefox.desktop /usr/share/applications/firefox.desktop.orig	
$  nano /usr/share/applications/firefox.desktop	

OR - for just, you:
$  mkdir ~/work
$  nano ~/.local/share/applications/firefox.desktop

Insert the following:
[Desktop Entry]
Version=1.0
Name=Firefox Web Sandboxed
Comment=World Wide Web
GenericName=Web Browser
Keywords=Internet;WWW;Browser;Web;Explorer
Exec=firejail --seccomp --private=~/work --dns=1.1.1.3 --dns=1.0.0.3 firefox %u -no-remote
Terminal=false
X-MultipleArgs=false
Type=Application
Icon=firefox
Categories=GNOME;GTK;Network;WebBrowser;
MimeType=text/html;text/xml;application/xhtml+xml;application/xml;application/rss+xml;application/rdf+xml;image/gif;image/jpeg;image/png;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp;x-scheme-handler/chrome;video/webm;application/x-xpinstall;
StartupNotify=true
Actions=secure-mode;private

[Desktop Action private]
Name=Private Mode
Exec=firejail --seccomp --private=~/work --dns=1.1.1.3 --dns=1.0.0.3 firefox %u -no-remote -private-window

[Desktop Action secure-mode]
Name=Super Secure Mode
Exec=firejail --seccomp --x11 --name=SecureMode --private=~/work --dns=1.1.1.3 --dns=1.0.0.3 firefox %u -private-window -no-remote -safe-mode

To Save, do: ctrl+x, then press Y to save. Enter to accept filename...

Once saved, reboot and Copy Launcher Icon to Launcher Tray... Try dragging and dropping (from Show Applications, FireFox) to your sidebar....

Note: All access for FireFox will be JAILed to the path ~/work and will not be able to get out of that Jail! Cool, huh.

You may access downloads via: $ nautilus ~/work/Downloads &

Or Open Files and goto : work/Downloads

Now, if at any time you get spy/malware/ad toolbars or whatever you do not want....

All you need to do is remove the work folder from your home path, all setting will be wiped along with bookmarks!!!

Backup/Export your bookmarks, then rm -rf /home/yourUSERnameHere/work