Till now in Firefox, to close a tab by double click you need to be using an extension. Which no longer required in upcoming Firefox 61 version as Mozilla now offers a preference in about:config which when toggled, a double click with mouse left button will close the selected tab.
Close tab by double click in Firefox Quantum
Ensure you’re using the latest Firefox beta which is at version 61,
1. Visit about:config
2. Search for browser.tabs.closeTabByDblclick and change the value to true
3. Start to double click on tabs, one by one, to see them closed.
What happens behind the scenes is a dblclick event is fired when you click on a tab twice with mouse left button.
For more details, check this Bug 1435142 – Pref to enable close selected tab by dblclicking it.