Do you have the problem on all sites, for example, when using the text box on this site, or only in more sophisticated editors?

Sometimes add-ons will take over a keyboard combination (not very considerate of them), and this can be hard to track down. As a quick test, you could try Firefox's Safe Mode. That's a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: Use Troubleshoot Mode in Firefox.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference?

Thanks you, yes, I have the problem on all sites. I needed it when I want to cancel the operation. I will try Safe Mode

I think Firefox only uses Ctrl+z to "Undo" typing or editing of text. Have you used it for other things in the past?

Yes, I used exactly cancel the action, open accidentally closed the scroll card. I tried Firefox's Safe Mode, but nothing has changed. Sorry for not perfect English.

I found another opportunity to cancel the operation - this is Ctrl + Shift + T. Thank you

Note that Ctrl+Shift+T is the shortcut key to open the last closed tab (History > Recently closed Tabs).

Thank you. In the browser I usually need this function - to open the last closed tab. Previously, it works with Ctrl + Z, but now it does not, although on MO programs it works.