We don't yet have either process-per-tab or other clues from Firefox/the embedding about whether the tab is in the foreground or background. But we can use timing information, Bug 1532838 increased the minimum nursery size to improve performance. We could do this only when minor GCs are fairly frequent. When they're infrequent it'd be quite easy to reduce this a lot, that's what I propose to do here.
Later when we have process-per-tab and know that we're in the background this feature will still be useful for foreground but not-very-busy tabs.
There's no point implementing this until after Bug 1506733 because it's not making the not-used memory available to other processes yet.