Memory leaks can cause Firefox not to release memory that it is no longer using, especially with older versions. There has been a lot of effort to reduce the leaks in recent versions, and Mozilla developers have have created tools to detect them.
[5] [6] To minimize leaks, you should upgrade to the most recent version. The most common memory leaks appear to be fixed in Firefox 2.
[7] Firefox 3 will likely use even less memory than Firefox 2 due to more memory leak fixes and further efforts to reduce memory usage.
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