If a website can be added as search engine in Firefox browser, Firefox 56 and earlier versions, used to show a magnifying glass icon in the search bar to tell the user that the site can be added as a search engine to the browser. Since Mozilla has removed search bar with Firefox Quantum 57 and merged into the address bar, the ability to add search engine discovery to location bar is not available till now, the version 61 has got that.
READ: Firefox 57 : How to add Search bar to Toolbar
How to add search engine to address bar in Firefox 61 Quantum
1. Visit the website in Firefox browser which you want to use as search engine.
2. Click on three dots (…) icon in the address bar to open Page Action menu and select ‘Add Search Engine’ option.

3. Done. Visit menu > Options > Search options to see the search engine added under one-click Search engines list.
Tip: Right click on ‘Add Search Engine’ Page Action menu item and select ‘Add to address bar’, so next time when you visit a search engine site, Firefox shows an icon in location bar informing you that search engine is available. Doing this, avoids the need to open Page Actions menu again and again.

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