This site cannot be fully signed out of

[Chai-T-Rex]

I signed into this site. It offered me a few OAuth options, and I logged in using my Firefox Sync account. I signed out because I wanted to use a different account. I clicked sign in and it didn't let me choose how to log in, it just immediately relogged me in using my Firefox Sync account. Clear cookies and site data doesn't fix it. Restarting the browser doesn't fix it.

How can I fix this?

[dftf]

Same: I've got to my profile-icon in the top-right and done "Sign Out", and then after have even tapped the shield 🛡 in the web-address bar and done "Clear cookies and site data", but still every time I tap "Sign in", it just goes straight back into my account here!

Not good...