FINALLY I found the solution to OS X not tabbing to dropdown fields in web forms.
Just Click All Controls
I found it by blind luck a few days ago while sifting through System Pref panels. Selecting “All Controls” in Keyboard -> Keyboard Shortcuts will force web browsers to tab (highlight) dropdown menus in forms on web pages. The normal behavior is to SKIP dropdowns for some INSANE reason. All the other form elements could be tabbed to, but not dropdowns. This was a highly irritating behavior, one of the few in Mac OS X that had me wanting to scream. Now that I found this, I don’t have to reach for the mouse when I fill in my State.
JFC that was a pain in the ass for about a decade.
Downsides
The downside is it’s an all-or-nothing setting, meaning I now get to tab through BS I didn’t ever want to, like settings when I create a Mail.app message.
Whatever. I’ll take the tradeoff.
