Thunderbird is an email client and you can work with it in the following way.
1. Open Mozilla Thunderbird.
2. In the left panel you can see a list of your email accounts. Double Click on "Inbox" or if its hidden, click on your email, and it will expand and show the inbox, the trash list. etc.
Anther option is to right click on the email and click on "Get Messages".
Thunderbird also automatically checks email every 5 to 10 minutes. It can also have new emails 'pushed' to it immediately in many cases. It also automatically checks mail when you open the program.