I posted this in another thread and made some edits to it, but then it got deleted when I tried to save the last edit for some reason. Anyhow, here's a revised version of that post and hopefully this won't be deleted.
I recently ran into an issue with a basic auth prompt that wouldn't play nice with 1Password X. For those interested it's the admin portal for a Communigate email server.
After writing a short novel here while continuing with the troubleshooting I figured out that my issue had nothing to do with 1Password X, the website in question or Firefox. Instead it turns out it's two of my browser add-ons which are causing it.
If I disable TC and remove the site from the Work container in FFMAC so the site opens in just an ordinary tab in Firefox everything works. But if I configure FFMAC to always open the site in the Work container it doesn't work. Same thing if I disable FFMAC and activate TC. When Firefox tries to open the site in a specific container it creates a new tab and that somehow seems to mess with 1Password X trying to automatically log in.
This is more or less the definition of an edge case so I'm not counting on it being fixed, and I'm not really sure if you can do anything about it or if the issue really lies with Mozilla.
1Password Version: 7.6.781 (Beta)
Extension Version: 1Password X 1.21.0 (build #20120)
OS Version: Windows 10 1909 (18363.959)
Sync Type: 1Password