Type about:accounts<enter> in the address bar.

[v57+] https://accounts.firefox.com/settings

Is this the button that is not working?

Make sure you are not blocking content.

Diagnose Firefox issues using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}acking-protection-firefox-desktop

Thank you very much, FredMcD. I tried all suggestions you made. Sorry to say that nothing worked.

I will try to create a new sync-account. Maybe these old entries will not be there.

I have another Firefox-account, created a few years ago. Login worked and when logging an old entry out, Firefox sent an code to my e-mail. With this code the entry got deleted. But not the other old entries. Now it is the same problem as before. Nothing is deleted anymore.

Now I will create a new account.

It is not possible to create a new account. I am giving up now.

I called for more help.