Thankfully Apple made this into a very easy system preference.
Open up the System Preferences tool and then select Keyboard & Mouse, and then Keyboard Shortcuts.
You’ll see the section at the bottom that says “Full keyboard access”.
Why Doesn't Tab Work for Drop-down Controls in Firefox on OS X?
If you are a recent Mac adopter and are frustrated by the weird tab behavior in OS X, you aren't alone. By default the Tab key only works for text boxes and lists, but excludes select controls (drop-down menus) on a form when using Firefox.
