There has been overflow menu under the ‘>>’ icon since a long time in Firefox browser. That appears on toolbar only when your browser window is narrow to display all toolbar icons. Till now to save space on the toolbar or to make it tidy and clutter-free, you used to move unnecessary icons to old hamburger menu or customize window, which not the case in Firefox 57 as the photon menu is no longer customizable.
In Photon customize mode: The overflow menu will be shown for the placement of icons and it has been set as destination instead of hamburger menu.
You can now pin icons to overflow menu permanently and unpin them later whenever you want.
When you upgrade from Firefox 56 to Firefox 57, which ships Photon UI, all icons (including webextensions’) that existed on hamburger menu will show up in the overflow menu.