My first reaction was: WOW. Ok, a browser reaching a million is kind of normal, as it is something that everyone will use when using a computer. But an email client? Hitting a million is a probability esp with corporations using Thunderbird but in a mere 10 days? Thus, WOW. You are doing a great job, Mozilla. Kudos to the Thunderbird team esp Scott MacGregor & the rest. Comes with 21 languages too. WOW.
Now we have 2 projects out released as 1.0 which are getting the headlines. That’s Firefox + Thunderbird. With 2 great “Edging” sites, TBE & TRE. There is hope as (hopefully) Mozilla expands to include more developers under its hood to continue on projects like Sunbird (Calendar) and Nvu (WYSIWYG editor) (Again hopefully, though improbable). They already have developers such as Daniel Glazman for Nvu, but it’ll be great if more joined in. Not to forget (hopefully) The Shining Edge (Sunbird) or The N-View Edge (Nvu). Anyway that’s enough of dreaming. 
Here’s some articles + reviews to read over @ these few sites:
JoeS | 19-Dec-04 at 11:21 pm | Permalink
That is a great number for sure. TB is a good product but (my opinion) not a great product until the devs can find the time to spend on some of the HTML compose isues that have been untouched for a very long time. Even given some folks revulsion for the use of HTML in email, there still remains a large market for Newsgroup user that likes to dabble in multimedia presentations.(stationery groups if you want to use that description)There is also a potential market for the HTML newsletter composer.
raananb | 24-Dec-04 at 3:46 pm | Permalink
Copernic Desktop Search for Windows, available at no cost at http://www.copernic.com, is a very efficient program, much more useful than the lackluster and very restricted recent Google product.
However, the current version of CDS only processes mail in Outlook & Outlook Express.
Hopefully, encouragement from the Mozilla user community will be helpful towards the inclusion of Thunderbird (and Mozilla) mail.
Please contact Copernic and ask them to extend CDS coverage to Thunderbird.
Dilissa | 27-Dec-04 at 12:32 am | Permalink
I love Thunderbird; it’s beautiful and much more useful than Outlook Express. No shock to me that it has been downloaded that many times. Keep up the good work.