see also https://wiki.mozilla.org/Performance/MemShrink
MemShrink is a project that aims to reduce the memory consumption of Firefox (on desktop and mobile) and Firefox OS. There are three potential benefits.
1. Speed. Firefox will be faster due to less cache pressure, less paging, and fewer/smaller GC and CC pauses. Changes that reduce memory consumption but make Firefox slower are not desirable.
2. Stability. Firefox will suffer fewer aborts/crashes due to virtual or physical memory exhaustion. The former is mostly a problem on 32-bit Windows builds with a 2GB or 4GB virtual memory limit, the latter is mostly a problem on mobile devices that lack swap space.
3. Reputation. Fewer people will complain that Firefox is a memory hog and that Mozilla ignores memory usage.