I have add a screen shot. You can see that the buttons are greyed out after the klick on OK.

Hello,

can anyone please give me an email address from the support? This problem is very annoying.

Other sites dealing with Firefox issues are mozillazine and reddit. I believe this is the only support facility offered by Mozilla. I never have that problem with the dialog you mention. Does it still happen if you start Firefox in "troubleshoot mode"? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode

Are you possibly using Private Browsing mode ?

Make sure you do not run Firefox in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History).

• https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history
• https://support.mozilla.org/en-US/kb/common-myths-about-private-browsing

• Options/Preferences -> Privacy & Security
  Firefox will: "Use custom settings for history"
• remove checkmark: [ ] "Always use Private Browsing mode"

@TerryN21

I have test it just now. The same.

@cor-el

No, I use the normal version. This problem is not every time, but very often. I use "Clear Recent History" very often.

Thank you, that you remove the [x].

The problem wasn't fix. How can I close the windows now without the [x]-button?

The "Clear Recent History" history window has a Cancel (Abbrechen) button that you can click to close this window if the OK button isn't working. You can also use the Escape (ESC) key close this window. If you click OK then the window should close and the selected items should be cleared. If this still fails then there might be a problem with clearing the selected data and you can try to only clear one item at the time. Note that active logins are about websites that use basic authentication (pop-up window to enter a username and password) like in the past used by FTP websites and not about regular logins that use cookies to remember/recognize you.

Does this only happen if you select the full range to clear everything ?

Maybe check the Browser Console to see if errors are reported.

• https://developer.mozilla.org/en-US/docs/Tools/Browser_Console

On the screenshot you can see, that the window is block. You have to wait long enough, then the window will be closing. Sometimes 30-60 sec.

Since replying earlier, I found with Firefox 91 that I had the same issue if I selected 'Everything'. If I selected 'Today', the window closed when I pressed OK.