I've observed this recently on both Windows (Firefox 112.0.1) and Ubuntu 18.04 (Firefox 112.0.2 Snap). Another symptom is that the "Customize your settings" button doesn't work. In both cases, it appears to be related to the DuckDuckGo Privacy Essentials add-on - disabling that (temporarily - it can be re-enabled after making the selection) seems to fix it. I also run UBlock Origin and Privacy Badger but neither of those seems to be involved.

I accidentally created two accounts; how do I merge them?

'multiple responses that say "This question has been asked before", but no links to previous answers like we're stupid for asking it' Can you provide a link to a question where this has happened?

It's encouraged: see Can I answer my own question? in the Help Center

See 50 Reputation need to comment and the links therein

Does this help? I accidentally created two accounts; how do I merge them?