mozilla and installing java plugin with xpi extension

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