The search box in Firefox is that box for searching (boy, definitions don’t get any better than that) usually located in the upper right of your browser window. By default you see it set to Google, but did you know you can use others that are listed there or install more of your own?
To use the search box, all you need to do is type in a phrase you’d like to search for into the box, and then hit the “Enter” key on your keyboard. From there you’ll be taken to a Web page with search results as you would see by going to the search engine’s Web site and doing it directly.
If you want to choose one of the other search engines you have installed, just click on the little square logo next to the search engine icon and you should get a drop down box with all the search engines you have installed. Pick the one you want to use, and then its icons should be shown beside the search box. To switch back, follow the same steps.
If there is a search engine that you need that doesn’t come with Firefox by default you can install it yourself to use. You can find a list of popular search plugins here. For even more choices, check out the Mycroft Project. If you want to make your own – this Web page is a good place to start learning how to do just that.