Because you haven't copied your plugins from the default /usr/lib/mozilla directory to the new Firefox directory. Anyway, FC6 is very old and unsupported, you really should get up to date with at least Fedora 8. F10 should be out sometime on or after November 25th.
If you really must use FC6 then get rid of the Firefox tarball you just downloaded and install the RPM with YUM from here:
http://blog.famillecollet.com/pages/Config-en
Wayne