Firefox Screenshots in current Nightly and in a future version, can be activated with a keyboard shortcut: Ctrl+Shift+S on Windows and Linux, Cmd+Shift+S on Mac, till now Firefox allowed users to launch the feature by selecting ‘Take a Screenshot’ option present in Page Action menu and web page right click menu.
On the other hand, the changes which Mozilla announced for Firefox Screenshots regarding removal of uploading functionality has been landed in Firefox Nightly, Yes! The Screenshots UI no longer shows save button to upload screenshots and ‘Download’ label has been added to existing download arrow which now shows ‘Download Screenshot’ message when you hover over it.

Before assigning the shortcut to Screenshots, the shortcut was already used to open devtools debugger on Windows and Linux, the company has removed the Ctrl+Shift+S shortcut for debugger and this change will come into effect with Firefox 66 onwards. Firefox makers have not assigned any new shortcut for the devtools debugger, as they need to assess how many actually used the shortcut in the first place from telemetry data before offering a new one.

As told by Mozilla, the company is working to alert uploaders who already saved shots to cloud that screenshots server is going away and to offer an option to export them.
Related articles:
Firefox Screenshots gets Annotations feature
Firefox Screenshots now works in Private browsing mode
Firefox yet to fix Screenshots misleading ‘Save’ button label
Firefox 55: Enable Screenshots feature
This stuff is for nerds. I did not understand many of it. When are we going to get user friendly instructions.
You saved my day! Thank you so much