One of the features I like most when I use Google on my mobile phone is that I can type the number of the search result instead of selecting it manually. The desktop version of Google.com doesn't show the order number for search results and doesn't offer any keyboard shortcut that could save you time.
If you install Greasemonkey for Firefox, you can add a script that sets some keyboard shortcuts for Google search. The script works even for the localized versions of Google.
You'll see that each search result has a digit in front of the title. This works only for the first 10 results from a results page, so it might be a good idea to stick to the default number of search results (which is 10).
You can also go to the next page of search results by tying n and go back to the previous page by typing p. To edit the query you have two options: either type e to select the query, so when you type something the query will be replaced, or type a to append something to the query.
A very cool feature is the tab mode: if you type t, the numbers will be highlighted in green and search results will open in new tabs.