mozilla and installing java plugin with xpi extension

[Tony Young (ty)]

All
I recently installing Mozilla 1.2.1 and after going to my netbanking site was told that I needed to install the java plugin.
I dutifully went through the indicated steps to find the Java plugin on netscape's site and selected the download button.

I received a popup with the option of Opening using application, or saving the .xpi file to disk.
I was under the impression that .xpi files indicated a relationship with XPInstall and that Mozilla/Netscape would handle this internally?

Because I couldn't really do anything else I saved the file to disk, but don't know what to do next.

One thing I noticed in Edit->Preferences that I though was strange was that there are no helper apps listed. I expected to see references for quicktime, flash, shockwave, and a few others for which plugins were successfully installed.

Can anyone enlighten me as to what to do next and what's gone wrong? (if anything)

Thanks in advance
ty