Right click on the upper right of the toolbar and you will see "Customise..." and click on that and it will bring you to "customise firefox".
In firefox 57 with photon for linux it looked like:

Maybe I'm sorry I updated, but now in Firefox quantum 61 it now appears like the following. I've only included the right window that shows overflow menu:

Does anyone know where this image would be found?
The following is another similar image that has appeared in the latest firefox versions, this page shows when there is a problem loading a webpage due to server problems:

Just wondered where these anime images are located. I don't see anything in .mozilla/default/prefs.js file. Yeah, they kinda creep me out.
Edit: Marked as solved. Thank you to mozillazine org and Brummelchen and morat for allowing me to ask such a question and give such quick and helpful advice/answers.