Dick,
Yesterday, I installed the excellent "MR Tech Local Install" extension
which, I believe, Aaron recommended in another thread:
https://addons.mozilla.org/extensions/moreinfo.php?id=421&application=firefox
I just now installed the Calendar extension in Firefox:
https://addons.mozilla.org/extensions/moreinfo.php?id=1270&application=firefox
(just to make sure we're talking about the same thing) which is updated
only to FF 1.5b1. As I installed it, the "MR Tech Local Install" extension
asked me if I wanted to "Override 'maxVersion' compatibility checking" and
I placed a checkmark in the box so as to do this. Then I shut down Firefox
1.5 and restarted it. When I look in the "Tools" menu there's an entry for
"Calendar" and selecting it opens a nice little calendar box. Clicking on
the "History" button sends me to a page where there is "This Day in
History" information.
Admittedly I haven't installed the extension in Thunderbird, but at least
I got it to work perfectly in FF.
I'm using Millennium Edition by the way.