Firefox 7 now uses much less memory than previous versions: often 20% to 30% less, and sometimes as much as 50% less. This means that Firefox and the websites you use will be snappier, more responsive, and suffer fewer pauses. Mozilla engineers started an effort called MemShrink to improve Firefoxs speed and stability by reducing its memory usage. The benefits are most noticeable if you do any of the following: keep Firefox open for a long time; have many tabs open at once, particularly tabs with many images; view web pages with large amounts of text; use Firefox on Windows;. use Firefox at the same time as other programs that use lots of memory.”]
Source: https://hacks.mozilla.org/2011/09/firefox-7-is-lean-and-fast/