Firefox provides a lot of built-in search tools, but using them requires a two-step process of typing search terms and then selecting a specific engine to use. There is a built-in contextual search that lets you highlight words on a page and right click to search, but you're limited to Google results using this feature.
Enter ConQuery, a nifty Firefox plugin that gives you access to all of your installed search plugins via the right-click context menu. This makes it easy to launch a search from any page on the web with any of your favorite engines with just a few quick gestures. See today's SearchDay article, Contextual Search with Firefox for a closer look at this powerful search tool.