IceCat doesn't show up under 'Add/Remove Applications' but is in the
repository. You can install it from Synaptic Package Manager, or in a
terminal$ sudo apt install icecat
If you are using Trisquel 7, Thunderbird is called Icedove. Again, it can be
installed from Synaptic Package Manager, or in a terminal$ sudo apt install
icedove
In general, if you don't see something in 'Add/Remove Applications' that you
suspect should be there, check Synaptic Package Manager.
If by Tor you mean Tor Browser, it is indeed not in the repository. Users of
Tor Browser need to receive updates immediately and simultaneously,
regardless of which distro they use, to ensure that they have identical
browser configurations, preserving anonymity. Download Tor Browser directly
from
https://www.torproject.org/projects/torbrowser.html.en
After downloading the tar.xz file, find a good place to extract it. I have a
directory in my home folder called Applications for applications like these.
After extracting, you now have ~/Applications/tor-browser_en-US (~ is your
home folder, equivalent to /home/[your username]). You can delete the tar.xz
file now.
Enter ~/Applications/tor-browser_en-US, double click 'Tor Browser Setup', and
it will turn into 'Tor Browser'. You can now double click that to launch Tor
Browser, no need to install.
If you would like Tor Browser to show up in menus alongside your other
applications, you'll need to create a desktop file$ sudo nano
~/.local/share/applications/tor-browser.desktopcontaining[Desktop Entry]
Name=Tor Browser
Exec=/home/[your
username]/Applications/tor-browser_en-US/Browser/start-tor-browser
Icon=/home/[your
username]/Applications/tor-browser_en-US/Browser/browser/icons/mozicon128.png
Type=Application
Categories=GNOME;GTK;Network;WebBrowser;In the paths following 'Exec=' and
'Icon=' replace '/home/mason/Applications' with wherever you extracted the
tar.xz file. You might have to logout and login again before Tor Browser
starts showing up in menus.
