best place to install programs?

Where is the best place to install your tars or rpms? My question comes from installing firefox. I did a look around the system and the firefox was installed in the /opt. At least that is what I saw.

A nice (not already dense populated) place is /usr/local/ that also has a /usr/local/bin/ for common links|scripts etcetera that you add yourself and want to have an overview of. But be sure to use the system managment installer for 'native' packages (packed in that way) and use this option for 'foreign' software only ('tars' etc.). My Firefox comes in an ordinary deb package. I suppose an rpm package also should be integrated into the system database by installing it as designed in the package. -- In my /usr/local/ is, for example, Realplayer and Java, which I couldn't find as Debian deb packages.