Sorry, I was a little flippant there. My point was that with its default settings, Safari does not have this problem. And nor does Camino, or Firefox for Mac. It's up to the browser to choose which elements in a page can get keyboard focus, and focusing *every* element (as Firefox does by default) isn't necessarily the best choice.
Aha! This means if you use Firefox, there is a workaround, as described in <http://kb.mozillazine.org/Accessibility.tabfocus>:
1. Go to <about:config>.
2. Set the value of "accessibility.tabfocus" to 1.
3. Return to translating.