[MinimizeToTray] Question about MinimizeToTray - Thunderbird Extension

[Beverly]

I have Thunderbird v1.5 and have recently tried to
install the "MinimizeToTray - Thunderbird Extension".
Any idea why it *only* installs to Firefox instead of
Thunderbird?  I've tried re-installing several times,
even making sure Firefox is shut down, to no avail.  

For some reason, the extension will not save to the
extension manager in Thunderbird, however, there's no
problem with it going to Firefox (which is exactly
where I don't want the extension).  Thanks for your assistance.