Hi, it's a new feature - https://screenshots.firefox.com/

If you don't want it, you can right click it, or in the 3-bar menu Customize mode, you can drag it to a different location.

If your question is resolved by this or another answer, please take a minute to let us know. Thank you!

thanks scribe. But why did it suddenly appear on my firefox menu? I find that very distasteful.

It's being rolled out gradually to FF55 users. Contributors - like me, have no say on feature changes, but I suppose if it didn't appear some time, users probably wouldn't be aware of its existence.

If you want to remove it completely, type about:config into the address bar, press Enter, accept the warning, scroll down to extensions.screenshots.disabled and double click on it to change 'Value' from False to True.

If you are worried about changes to FF, be aware that in v57 - due for release in mid-November, there will be major alterations to the UI (appearance), and all current add-ons will stop working unless their developers have ported them to WebExtensions. In case you want to search for more info, these changes go under the name of Photon.