Firefox by default provides zoom widget in menu, which allows user to adjust Zoom level — increase (Ctrl++) or decrease (Ctrl–) or reset zoom level (Ctrl+0) –for a page, but Firefox primary UI no way indicates or alerts users when a site zoom level is in non-default state, to address this, Mozilla has added Zoom indicator to the URL bar in Firefox 51, which will be shown only when the page/site Zoom level is not 100%.
With the zoom number shown in address bar, users can able to quickly click on the icon to reset zoom level to default and indicator goes hidden at that time with this action.
READ: How to Reset Websites Zoom Level to Default in Firefox browser
