Firefox 7 uses less memory than Firefox 6 (and 5 and 4): often 20% to 30% less, and sometimes as much as 50% less. In particular, Firefox 7′s memory usage will stay steady if you leave it running overnight, and it will free up more memory when you close many tabs.
This means that Firefox 7 is faster (sometimes drastically so) and less likely to crash, particularly if you have many websites open at once and/or keep Firefox running for a long time between restarts.
Firefox has a reputation for being a memory hog, and the efficiency with which it uses memory has varied over the years. For example, Firefox 2 was quite bad, but Firefox 3, 3.5 and 3.6 were substantially better. But Firefox 4 regressed again, partly due to a large number of new features (not all of which were maximally efficient in their first iteration), and partly due to some over-aggressive tuning of heuristics relating to JavaScript garbage collection and image decoding.
As a result, Mozilla engineers started an effort called MemShrink, the aim of which is to improve Firefox’s speed and stability by reducing its memory usage. A great deal of progress has been made in only 7 weeks, and thanks to Firefox’s new rapid release cycle, each improvement made will make its way into a final release in only 12–18 weeks. (These improvements are available earlier to users on the Aurora and Beta channels.) Firefox 7 is the first release to benefit from MemShrink’s successes, and the benefits are significant.
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