First of all, don't forget to ask your sysadmin to setup autodiscover, as it will make the process of setting up Thunderbird much easier.
Go to Tools -> Account Settings and click on New
Click on Next
Enter the user's full name, his e-mail address and password. Click on Next
Outlook will search for the web service...
Allow autodiscover.emailarray.com/AutoDiscover/AutoDiscover.xml to configure the client.
The account is now configured. Click on Finish