Thunderbird supports running from removable drives with minimal reconfiguration. Repackaged Thunderbird as a complete, removable drive-friendly email client. This grew out of a mozillaZine forum topic on making Firefox portable back in June of 2004. This is an unofficial package. It is an official build, but has been modified and a specialized launcher added to make extensions portable. No warranty is expressed or implied.
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Download: Portable Thunderbird 0.9
News source: JohnHaller.com