First off, the current version of Firefox in Slackware 14 is 18.0.1. So are you using FF 15.0.1? Or did you mean to type FF 18.0.1?
If you are using FF 15, you should upgrade to FF 18.0.1 ASAP.
In any case, here is how I installed flash 11.2 on Slackware 14 as root:
# tar -xvzf install_flash_player_11_linux_xxx.tar.gz -C /usr/lib64/mozilla/plugins/
Note that I am using 64 bit Slack 14. So if you are using 32 bit Slack 14, the path will be /usr/lib/mozilla/plugins/
This will untar the flash .tar.gz and copy the /usr
directory and libflashplayer.so
that are created from the flash .tar.gz to your /usr/lib/mozilla/plugins/
Remove the flash stuff that you copied to /usr/lib/firefox-XXX/components/
directory and the stuff you copied to your /usr
directory first to avoid potential conflicts.
There is also a slackbuild for Flash that you could use for this: http://slackbuilds.org/repository/14...player-plugin/