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Thunderbird add-ons that I've either used for years, or tested and highly suggest.
nothing ever works on ffox
Pixie's favorite lightweight themes for Firefox and Thunderbird.
Featured Personas for Thunderbird
Here are some great add-ons for Thunderbird
The add-ons included in this collection are all ones that are part of the messaging category on Mozilla Labs: https://mozillalabs.com/messaging/