How to use the Tab key to navigate to a button or select element on a Mac?

How do I use the Tab key to navigate to a button or select element in Firefox? I've tried setting the tabindex attribute to no avail. I'm on a Mac.

Solution

If you're using a Mac, you need to open the Keyboard preference panel, switch to the Keyboard Shortcuts tab and select:

"press Tab to move the keyboard focus between: (x) All controls"