A great feature in Firefox is the search box, which let's you send a search to almost any engine without loading the front page. I can normally press Ctrl+K, type the terms, hit Enter, and jump to Google's results.
However, I also have search plugins for Acronym Finder, Webster Dictionary, Webster Thesaurus, and, most importantly, Wikipedia. Switching engine, I had thought, entailed moving my hand off the keyboard and onto the mouse, then pointing at the top-right corner, clicking, selecting the engine, moving my hands back to the keyboard, typing the terms, and only then hitting Enter and being taken to a page of results.
No longer! When the search box is focused (Ctrl+K'd,) use Ctrl+Up or Ctrl+Down to scroll through search engines. Useful, eh?