Mozilla's MemShrink project is an ongoing effort to reduce Firefox's memory consumption, something that a lot of users frequently complain about.
With the upcoming Firefox release, we can expect a major improvement to the browser's memory consumption particularly for users who use 3rd party add-ons with their favorite browser.
Leaky add-ons will be better handled in Firefox 15. While Firefox gets flak for high memory consumption, leaky add-ons frequently cause the biggest impact. Finally, we'll be seeing improvements in this department.