If it's there (AND) if it's version 2.0.0.18, that might be the
easiest way to go. But installing from the tar.gz is really simple
and you know where the package and all its components are located,
so it's easy to install plugins manually.

I just took a quick look at my /usr/local/mozilla and I have only
two plugins installed, the latest Java runtime and flash, and
those were manual installs.

For some inexplicable reason, I've never been able to get auto
installs of addins/plugins to work on any of my Linux or UNIX
boxes, and it offers to download the plugins and then I have to
manually install them (which is a one-time only thingie -- it's
either s symbolic link to the *.so or a *.so itself).

Auto plugin install works fine on Windows, though.

Have you installed any plugins and, if so, were they all a point
and click install from within Firefox?

Note that whichever route to install firefox 2 you choose, make
sure it's 2.0.0.18 and not 2.0.0.17 (some security issues).