Context Search is a Firefox extension that expands the context menu's 'Search for' item into a list of installed search engines, allowing you to choose a specific search engine every time. Search engines can be added or reordered in Firefox's Search Toolbar ("Manage Search Engines"). Once the search engine menu is visible:

• Left click to search in a new background tab.
• Middle click to search in a new tab and switch to that tab immediately.
• Ctrl+click to search in a new tab and switch to that tab immediately. If you have Firefox set to switch to new tabs immediately, the above key combinations will have the inverse effect.

Shift + click opens search results in a new window. Hidden Preferences

• extensions.contextsearch.separatorItems: A comma-separated list of search engines. A menu separator will be added before each.
• extensions.contextsearch.quoteStringsWithSpaces: Automatically surround search strings that contain spaces with quotes).