hi, the feature will be rolled out to users of firefox 55 gradually over the next days and weeks. if you can't wait, enter about:config into the firefox address bar (confirm the info message in case it shows up) & search for the preference named extensions.screenshots.system-disabled. double-click it and change its value to false.

Thank you very much!

I already had it in 55 beta, now it seems updated to 55 release and not to 56 beta although I'm still in the beta channel. Maybe this caused it disappearing.

On the beta channel you're helping to test the release candidate builds. You'll receive an update to 56beta in the coming days :-)