Use the tab key to navigate through Google and DuckDuckGo search results
This extension adds a tabindex attribute to each search result link.
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. On Firefox, you may also try setting accessibility.tabfocus to true in about:config.
Also available as a userscript if you use Greasemonkey (gist here).