Starting from Firefox 51 the browser shows Zoom indicator in the URL bar for sites if zoom level is not 100%, if you don’t want to set zoom level to default for a site (sites) Firefox is showing and don’t want to see the indicator in location bar, you can disable or hide it. Unfortunately, there is no hidden preference present in about:config for this as you need to rely on Classic theme restorer add-on.

Disabling Zoom indicator in the URL bar in Firefox 51+

1. Once you’ve installed the latest version of CTR from AMO,

Visit add-ons manager >Extensions > Classic Theme Restorer >  Options  > Location bar

2. Check ‘Hide zoom button (if present)’.

3. Click OK.

READ: Mozilla removes Unified Complete Preference from Firefox

How to Restore Firefox’s Old Search UI and Preferences Window