Selected Search is a new add-on for Firefox that makes it easier to start a search from any page you're on. It works by taking text you've highlighted with your mouse, and then bringing up a small pop-up list of all the search engines you have installed. From there you just pick the one you want and it opens up behind the scenes in a new tab.
Firefox has its own built-in contextual shortcut that does this with whatever search engine you've got set up as the primary. The big difference with this extension is that you can very quickly pick whatever engine you want to search with depending on what you've highlighted.
What I really like about this tool is that you can continue to do all the normal things with highlighted text you'd normally do, including dragging it off into other programs or open windows, or using keyboard and contextual shortcuts to copy. It also does not always come up when highlighting text that's in a form, meaning you can keep it installed without interrupting your usual work flow, however this was hit or miss. It came up on certain form fields, but not in others.
Another tool that does the same thing with a little more visual flair is Drag and Drop Zones, which lets users drag highlighted text into a grid of search engines and keyboard shortcuts. It's a little more customizable, but may be a little harder to learn than this.