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Currently not working on ThunderBird 52.1.1 but I'm confident it will soon!

You should really open up some account for donations, or, get directly in touch with the dev team to correct this once for all, this is really an embarassing issue!

Kind Regards,
Alessandro Salvati

EDIT: Thank you for your answer! I still don't know why Thunderbird hasn't upgraded yet but your answer definitely fixed my issue :)

Best

Это отзыв для предыдущей версии этого дополнения (1.6.3). 

You can either use StartupMaster 1.6.2 with Thunderbird 52 or StartupMaster 1.6.3 with Thunderbird 53+. I've marked 1.6.3 as incompatible with Thunderbird 52, I'm not sure why Thunderbird allowed the upgrade in the first place. (You can downgrade to previous versions by following the "Show compete version history" link in the "Version information" accordion.)