You can check this pref on the about:config page and make sure it is default (false).

• extensions.screenshots.disabled = false

You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

You can find items like "Take a Screenshot" in the drop-down menu that opens if you click the "Page actions" button (3-dot icon at the right end of the location/address bar). You can right-click an item in the 3-dot "Page actions" drop-down list to add its button to the location bar for easy access.

Excellent! Thank you very much, that was the mistake. But I can't say why this was there all of a sudden !!

I have exactly the same problem. The difference is that this setting was "false" already and I still don't see this option in right-click menu.

Update. I experimented with other hidden prefs containing the same name but beginning with "devtools", there were 2 of those: both were false and I set their values to true. After I restarted Firefox the option was back. It's rather unpredictable.

Is not Ctrl-Shift-S working? You can grab a screenshot, a region/screen-object, an area, and even drag your pointer off-screen to grab an extraordinarily long area - scrolling.

To anyone having problems taking screenshots in Firefox: I'll close this thread, since ra.maier (the person who asked the original question) has indicated that the problem has been resolved.

Please see the Take screenshots in Firefox article to learn more about taking screenshots in Firefox, including how to add the "Take a screenshot" button to your Firefox toolbar in Firefox version 88 and above. If you need more help please ask a new support question. See Mozilla Support rules and guidelines.