To get Raindrop working, you have to disable the Tracking Prevention feature in the menu bar, under Develop. This is a massive shame, as privacy is one of Orion's best features, but I expect this also means the solution to this (and likely many login-persistance issues with other extensions) can be easily fixed by whitelisting some tracking essential to common extensions. In the meantime, disabling Orion's built-in tracking prevention and downloading a third party alternative that gives more granular control (uBlock, Privacy Badger, etc) will, I hope, do a similar job. Hope this helps someone that is as reliant on Raindrop as I am, and helps the Orion team expand their extension compatibility 🙂