browser.search.context.loadInBackground controls whether a search from the context menu with "Search <search engine> for <selected text>" opening a new tab will give focus to it and load it in the foreground or keep focus on the current tab and open it in the background.

Values

true
New tab with search results will be opened in the background, focus stays on the current tab.
false (default)
New tab with search results will be opened in the foreground.