/usr/share is not a very good directory for installing applications. If you are installing local applications the best directory is /usr/local (/opt is ok as well). So lets say you install firefox into /usr/local/firefox, you would create a symlink to the firefox executable as follows
ln -s /usr/local/firefox/firefox /usr/local/bin/.
That will make firefox run everytime a user types "firefox" into a terminal or creates a desktop shortcut with "firefox" as the command name.