Is it possible you're trying to install the software as non-root ?
Then you wouldn't be able to write in directories that are not globally writable (permission problem). This causes generally a message like:
mkdir: cannot create directory `/usr/foo': Permission denied
If you try to install actually being root, then probably you try to install into a subdir where the parrent doesn't exist:
like if you don't have
and you try to install into
It could be the installation fails to create that installation directory, since the parent doesn't exist. So check whether /opt (or whatever) exists, and whether you have write-access to that directory.
Good luck, and hope this helps.
AND in reply to your "locate firefox" list, is it possible your locate-database is obsolete ? (have you recently run updatedb (maybe needed as root )?)
My installation, but I fail to assure it was a "default" installation (can't remember it), returns firefox executable as
/usr/X11R6/bin/firefox -> /opt/firefox/firefox-sh -> /opt/firefox-0.9.3/firefox.sh