Tab Focus through Google Results v1.1.1
Tab to select the next search result.
Also available as a userscript if you use Greasemonkey (gist here).
Firefox for the Mac doesn't do tabbing by default:
On a Mac, this will only work if you can already tab through elements in a page. If it doesn't work, you will probably have to turn on Full Keyboard Access under System Preferences > Keyboard > Shortcuts, setting the radio to All controls. Alternatively, you may try setting accessibility.tabfocus to true in about:config