For office emails, I use Microsoft Outlook 2007 in office PC and Thunderbird 3 for home PC. Our email server is Microsoft Exchange Server I think. I've found that, Thunderbird 3 doesn't download emails that I downloaded using Outlook in my office PC. Is there any way to configure Thunderbird so that it download already downloaded emails?
Most probably, you are accessing your mail server via POP3. If that is the case, then your Outlook account might be deleting downloaded mail from your mail server. There is no way for Thunderbird to get it then.
To work around this, you should configure your Outlook to leave your mail in the server, then configure Thunderbird to delete the messages when you download them at home.
For more details, here's something from Microsoft: Leave e-mail messages on your e-mail server
The instructions are at the bottom of the article, but you have to click on the particular topic to expand them.