As of this writing, Thunderbird does not have a built-in Help Viewer. Navigating to Help | Mozilla Thunderbird Help routes you to http://mozilla.org/support/thunderbird, which is the online help site for the Mozilla Thunderbird mail client. If this site doesn't address your specific question, there are a number of other places to go to get help.
One great resource is the MozillaZine Thunderbird Support forum, located at http://forums.mozillazine.org/viewforum.php?f=39.
These forums require you to register, but after you post your questions, responses usually come fairly rapidly (sometimes within 24 hours). Because mail configurations can be rather complex, it is helpful to include as much information as you can in your post. This will definitely help the many forum watchers who are around troubleshoot your problem and get a solution posted much more quickly.
MozillaZine (http://www.mozillazine.org) also hosts several Thunderbird developer forums (http://forums.mozillazine.org/index.php?c=8) that may contain useful information, but the support forum listed previously is the better first stop. These forums are categorized into sections that cover Thunderbird builds, general topics, features, and bugs.
If you want to visit IRC (moznet, or irc.mozilla.org) and try to get some live help, you can visit the #thunderbird forum, but you are probably better off going to #mozillazine if you have Thunderbird questions.
You can also consult the Release Notes, which can be accessed from the Help | Release Notes menu option. The Release Notes may save you some frustration if something is not working as expected and you can't figure out why. Just like any software that is continuously under development, Thunderbird will have bugs, so don't expect perfection. But do expect a very active development community that is monitoring the Bugzilla bug database and is continually working to make the product better.
Third-Party Support Options
Remember that Thunderbird is distributed as a free product, and the Mozilla Foundation does not offer end-user support. There are a few options for paid support if you cannot get your questions answered in the forums or on IRC. For a list of these options, visit http://www.mozilla.org/support.
Other Online Resources
There are also a number of other online resources that you can consult for helpful information: