In Windows, in any windows form or web browser, you can use the tab button to switch focus through all of the form fields.

It will stop on textboxes, radiobuttons, checkboxes, dropdown menus, etc.

However, in Mac OSX, tab skips dropdown menus. Is there anyway to change this behavior, or access the above items mentioned, without using a mouse?

I have found that I also need to set accessibility.tabfocus to 7 in Firefox's about:config.

Go to System Preferences > Keyboard and Mouse, then choose Keyboard Shortcuts. At the bottom, ensure Full Keyboard Access is set to "All controls". It's a long time since I turned it on but I think that's all you need to do

Apple Menu > System Preferences > Keyboard & Mouse > Keyboard Shortcuts:

Change the radio button at the bottom from "Text boxes and lists only" to "All controls."

Edit: Dammit. We're a fast group around here aren't we? :-)

It's in the System Preferences - this blog post shows where the setting is.