error while loading shared libraries: libgtk-x11-2.0.so.0: No such file or directory
Your system is 64-bit, but since you want to run a 32-bit version of Firefox, you need to install the 32-bit version of the library:
sudo apt install libgtk2.0-0:i386
I was having the same problem. @fkraiem answer made me figure out another possibility that fix my issue.
I was trying to run a app in the wrong architecture. So make sure if you are trying to install the right app for your system a 32 or 64 bits.