There is an updated pull request by mmnmnnmnmm against master on the void-packages repository

https://github.com/mmnmnnmnmm/void-packages thunderbird
https://github.com/void-linux/void-packages/pull/24509

thunderbird: update to 68.12.0
Final release in the 68 branch, next release is 78, which contains big OpenPGP changes and supports only webextensions.

A patch file from https://github.com/void-linux/void-packages/pull/24509.patch is attached