How are you searching?

• searching via the location bar
• search bar on the Navigation Toolbar
• search bar on the about:home page
• other search bar (e.g. added by extension)

If you are searching via the Search Bar on the Navigation Toolbar, this preference can be changed to have searches there open in a Tab.

Type about:config in the Address Bar and hit Enter. Then answer "I'll be careful". Type this pref in the Search at the top.

browser.search.openintab = double-click to toggle to true

Thanks! That works.