By default, on a Mac in Firefox, tab moves from one form element to another—except it will skip radio buttons, checkboxes and drop-down select boxes. I have suffered in silence since I started using the Mac. I finally found out that this too can be remedied. Hooray!

There is an actual setting in the Mac OS that produces this behavior. To change this so that tab treats all form elements with equality go to: System Preferences -> Keyboard and Mouse and select the tab Keyboard Shortcuts. On the bottom you will see a setting for “Full Keyboard Access”. Just set it to All Controls. This setting will take effect right away, no need to restart Firefox.

Found this in the comments on John Resig’s blog