Unfortunately I find that when trying to authenticate with raindrop.io in this extension, the Sign In button takes me to a new FireFox tab.
I log in there, but the extension window doesn't recognise that I've logged in and just shows the 'Please log in to start' dialogue.
I'm a paying user, so really wish I could get this working. I've spent a lot of time tinkering with FireFox settings, but haven't been able to get it working.