From MozillaZine Knowledge Base
When text is selected in the browser, the context menu contains an entry to search for the selected text with a search engine. This preference determines whether that search opens in a new browser window or in a new browser tab.
Possible values and their effects
Open context menu searches in a new tab.
Open context menu searches in a new window. (Default)
SeaMonkey (nightly builds since 2006-08-11)
A checkbox labeled â€œOpen a tab instead of a window for a context menu web searchâ€ is located under Edit → Preferences → Navigator → Internet Searchâ€
First checked in
Has an effect in