The original XUL-based vertical tabs actually moved and restyled Firefox's native tabs, instead of creating a lookalike (which is all that's possible now). This meant that unrelated addons that did things like grey-out unloaded tabs, or alter the favicons to have some sort of indicator, still worked on the vertical tabs. The current vertical tabs addons all have to do it themselves or add some sort of API for the other addons to interact with to get the same effect.