Where is Ubuntu storing installed programs?

I am using Ubuntu and when I click on a program to download Firefox asks me "What should firefox do with this file?" And in the "Open with" I would like to find a program "Package installer". The problem is - I don't know where to look for. Where is the program stored (I installed it using Ubuntu Software Center)

They are usually stored in the following folders:

/bin/
/usr/bin/
/sbin/
/usr/sbin/

If you're not sure, use the which command:

~$ which firefox
/usr/bin/firefox

If you installed the package with the Ubuntu package manager (apt, synaptic, dpkg or similar), you can get information about the installed package with

dpkg -L <package_name>

Simply, type the command:

~$ whereis "program name"

for some applications, for example google chrome, they store it under /opt. you can follow the above instruction using dpkg -l to get the correct naming then dpkg -L to get the detail.

hope it helps