I like using Firefox, but the damn program + addons eats up RAM. Mine was up to using 600MB of RAM at one point and the browser has becoming slow as molasses. After some searching and experimenting, these changes below have helped a lot with reducing RAM usage.

1. Install Memory Fox
2. Modify registry

MEMORY FOX ADD-ON
https://addons.mozilla.org/en-US/firefox/addon/53880/

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MODIFY REGISTRY
STEP 1 - TcpAckFrequency - NOTE if you are running Windows Vista this setting may not have any effect - a hotfix is needed which i'm tracking down. This works fine under Windows XP

Type "regedit" in windows "run.." dialog to bring up registry menu

Then find: HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/Tcpip/Parameters/Interfaces

There will be multiple NIC interfaces listed in there, find the one you use to connect to the internet, there will be several interfaces listed (they have long names like {7DBA6DCA-FFE8-4002-A28F-4D2B57AE8383}. Click each one, the right one will have lots of settings in it and you will see your machines IP address listed there somewhere. Right-click in the right hand pane and add a new DWORD value, name it TcpAckFrequency, then right click the entry and click Modify and assign a value of 1.

You can change it back to 2 (default) at a later stage if it affects your other TCP application performance. It tells Windows how many TCP packets to wait before sending ACK. if the value is 1, windows will send ACK every time it receives a TCP package.