The Good Sign-Offs Are.na channel (https://www.are.na/meg-miller/good-sign-offs) maintained by Meg Miller and Hannah Barbera contains numerous sign-off messages ranging from conventional to witty to heartwarming, for your email writing pleasure.
When you click on the extension icon, the browser extension queries the Are.na API for a random message in this channel, then inserts the message to your cursor's position in your Gmail editor.
It will take a couple seconds until a message is retrieved. Currently does not work anywhere else than Gmail. Largely experimental work as of yet.
Download link for Firefox: https://addons.mozilla.org/en-US/firefox/addon/random-good-sign-off/
Browser extension is not affiliated with the Good Sign-Offs channel maintainers; in case you have technical questions or feedback, please open an issue on this Github repo.
Extension icon is from Feather https://feathericons.com/