When I got my first Mac (well, actually even before that when I was using my wife's Christmas present last year, which was our family's first Apple computer) I was astounded to realize that when using web forms (such as filling out a registration, or searching for plane tickets), hitting "tab" to advance to the next field worked great until you got to a checkbox or a dropdown. It would skip right over those! So no more signing into gmail and easily toggling the "remember me" checkbox with the keyboard. No more tab and "down", "down" when selecting my expiration date when purchasing something.
I did some searches a year ago (apparently not very good ones), and came up empty. Of course I didn't try that hard.. I just continued on with whatever I was doing. But every time I would hit a form where I wanted to tab to a drop down, I got more and more irritated. Given that the problem was both in Safari and Firefox should have tipped me off to the fact that it was a system setting somewhere, but I was beginning to think it was a hidden, hard-coded setting that I could not change.

Fortunately, there are some prominent blog posts these days (and Joel's wonderful stack overflow site with an answer). The answer is easy:
a) System Preferences > Keyboard and Mouse.
b) "Keyboard Shortcuts" tab.
c) Select "All Controls" in the bottom section about full keyboard access.