Glad to see they are finally doing something to address memory fragmentation besides suggesting that users "occasionally restart Firefox". Seems like this has been a major problem in Firefox forever.
Mozilla has been well aware of this problem for years. See this blog post from 2008, 3 years ago, detailing the changes that went into optimizing memory use for Firefox 3, including switching to jemalloc to decrease fragmentation: http://blog.pavlov.net/2008/03/11/firefox-3-memory-usage/
