FlashGot Download Link is 404-ish.
URL: https://flashgot.net/getit
On that page, that big green button, "INSTALL", points to AMO, where FlashGot (& all legacy extensions) have been banished from.
Oh, you could point it to TB, addons.thunderbird.net/en-US/thunderbird/addon/flashgot/.
FlashGot Download Link is 404
FlashGot Download Link is 404
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1.19) Gecko/20110420 SeaMonkey/2.0.14 Pinball NoScript FlashGot AdblockPlus
Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:56.0) Gecko/20100101 Firefox/56.0 SeaMonkey/2.53 Lightning/5.8