From MozillaZine Knowledge Base
For earlier versions of Thunderbird, Offline Support is an installation option that gives Thunderbird various offline capabilities found in Mozilla Mail (the e-mail component of the Mozilla Suite). Originally a Thunderbird extension, Offline Support was made part of the basic Thunderbird package in version 0.6. When installing Thunderbird, if you choose a "Standard" install, Offline Support will automatically be included in your Thunderbird installation. If you choose "Custom" install, you can choose not to include it in the installation.
In Thunderbird 1.5, the Offline Support functions are included with all installations, both "Standard" and "Custom".
- Thunderbird Offline Extension: some of the material is a bit dated (especially regarding the user interface), but this document gives a good overview of what Offline Support provides.