Just a few days ago I installed Thunderbird (since I love Firefox so much) to backup my Google Gmails through its PoP service.

Well, i entered pop server name, send server name, exactly according to insturctions provided so kindly by Mozilla and Google.

I am able to connect to Gmail through Thunderbird, but it doesn't want to download any more mails newer than February 15th or so of 2005. So, most of my Gmails I cannot download.

In fact, I didn't really want to download those old e-mails, I wanted to download newest ones first then get older ones.

Oh, I can send e-mails okay.

Anyone can shed some more light into this?

You need to change a setting in your Gmail, not Thunderbird. Log on to Gmail and go to Settings>Forwarding and POP>POP Download and check "Enable POP for all mail. That should fix your problem. If not tell me and I'll look into it further.