Overall memory efficiency is gauged by the difference between a browser's single-tab memory usage and final -39-tab memory management total. The browser with the lowest score is able to return the most physical memory back to the operating system without actually closing the application itself, but simply by decreasing workload (closing tabs).
Chrome takes the lead for memory efficiency on both operating systems, as Firefox takes second place on OS X and IE9 takes second under Windows 7. Third place on OS X goes to Safari, while Firefox takes third on Windows 7. Opera finishes last on both platforms.
The charts below are for the single-tab and 40-tab memory usage tests, followed by the -39-tab and -39-tab plus two-minute memory management tests.
The three cross-platform browsers all report approximately double the memory usage on OS X than on Windows 7. Firefox only gives back about one-third of its 40-tab memory usage total on OS X, a sharp increase from Windows 7.