I'm blind, so can't see the screencast. That said, tab/shift+tab alone aren't enough to navigate the toolbar buttons. In order to avoid cluttering the tab order (and thus to ensure an efficient experience), tab moves between "groups" of buttons. Once you reach the desired group, you use the right arrow to move to the button you want. For example, if the page actions aren't visible (no page loaded), with default configuration, you would press control+l, tab once to the Library button, then right arrow several times to get to the Firefox Accounts button.