It's one of the best features of Chrome - that you can simply type a search query into the address bar and it will display the results. Here's a site I found that makes Firefox mimic Chrome's address bar search functionality.
Hack to Modify Firefox 3 Address Bar to Return Search Results When Query or Invalid URL Is Entered
Essentially you change the keyword.URL setting in about:config to something like "http://www.google.com/search?q=", or something similar depending on what search engine you use.
