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Follow these simple steps to get your Firefox memory usage under control. These configuration changes are a trade-off between internet browsing speed and memory usage. They are most useful if you are multitasking and need Firefox to be less of a memory hog while using other applications.

1. about:config
Type “about:config” into your address bar to access the hidden configuration options for Firefox. (FireFox have 7 hidden pages!)

2. browser.cache.memory.capacit
Controls how much memory is used to cache pages.
Right click on the page and choose New then Integer and create an integer called browser.cache.memory.capacity and set it to a low number. This controls how many kilobytes of memory are used for the browser’s cache.

3.browser.cache.disk.capacity
Set browser.cache.disk.capacity to between 5000 or 15000.
128MB to 512M -> 5000
512BM to 1GB of RAM -> use 15000

4. config.trim_on_minimize (optional)
Configure Firefox to swap memory to disk when minimized. This will make Firefox less responsive if you minimize/maximize a lot, but it is good if you need to free memory while multi-tasking to other applications.
Right click on the page and choose New then Boolean and create a boolean called config.trim_on_minimize and set it to true. Now when you minimize the window it will free up memory.

5. network.prefetch-next
Network link prefetch will download all pages with the rel=”prefetch” tag. This uses up more memory than you would otherwise think.
Set network.prefetch-next to false.

6. browser.sessionhistory.max_total_viewers
This controls how many pages of history are kept in the back/forward buttons. Lower the value to something acceptable. If you do not use back/forward often then you can set this very low.
Set browser.sessionhistory.max_total_viewers to a low number.

Source: internetducttape.com