We do all sorts of aggressive throttling for background tabs, but we define "background" as "not currently selected in the tabbrowser". We should consider extending this definition to all tabs in windows that are not on the current desktop (e.g. with Spaces on Mac or multiple desktops under Linux WMs). Detecting this condition might be complicated, though.

I'm not a mozilla developer but I have a hopefully useful idea: is it detectable that the browsers window is completely obscured (no part is visible, not sure I have the right terms)? At least in X, I believe windows on different virtual desktops were so obscured. If being completely obscured is detectable and it applies to windows not on the current desktop, using this method of detection would have the additional benefit (as I see it) of marking as inactive the tabs in windows that are simply completely behind other windows. If this happens to be stupid, sorry for the bother. 8-)