In Safari, bookmarks in the bookmarks bar can be accessed with ctrl-1, ctrl-2, etc.

In Firefox, there are features built-in for using keywords to access sites quickly, but the above functionality isn't part of FF as such.

Enter Bookmark Keys https://addons.mozilla.org/en-US/firefox/addon/3647. This extension does this, but unfortunately isn't FF3 compatible.

Enter http://www.donesmart.com/sitelauncher/ or https://addons.mozilla.org/en-US/firefox/addon/10127. This actually goes beyond the Safari functionality, and lets you assign a combination of modifier keys to directly open a new tab with the bookmark, or bring up the list of all sites for which shortcuts exist. Check it out! (The only nag I've got with SiteLauncher is that by default it places a 'Site Launcher' item into the firefox menu bar, which takes up a lot of space.)