Tab Mix Plus add-on (not compatible with Firefox Quantum) lets you open urlbar and search bar results in a new tab, since the extension is not available for Quantum, users feeling the lack of it and want a preference to open everything from address bar in new tab that could be web addresses (URLs) or search suggestions.
Do note, Firefox already offers browser.searchbar.openintab preference to open search results in new tab, since the address bar and search bar were unified into one in Firefox Quantum, Mozilla now offers a new option in about:config in Firefox 60 (currently in Nightly), which, when enabled, lets you always open URLbar results in new tabs.
Make URLbar results always open in a new tab in Firefox
1. Visit about:config
2. Search for browser.urlbar.openintab and change its vlaue to true.
3. Restart the browser.
If you don’t know, Firefox 57 or later versions already include a preference to open bookmarks in a new tab.