There was overflow menu under the ‘>>’ icon since a long time in fire Fox browser. This appears on the toolbar only when the browser window is narrow to display all the toolbar icons. Until now, to save space on the toolbar or make it neat and tidy, you used to move unnecessary icons to the old hamburger menu or customize the window, which is not the case with Firefox 57 like photon menu is no longer customizable.
In Photon customization mode: Floating menu will be displayed for icon placement and has been set as target instead of hamburger menu.
Now you can pin icons to the floating menu permanently and unpin them later whenever you want.
When you upgrade from Firefox 56 to Firefox 57, which comes with Photon UI, all icons (including webextensions’) that existed in the hamburger menu will appear in the floating menu.
Firefox 57 Photon Integration Tour
Firefox Photon design updates