Firemin is a tiny tool that's been designed to minimise Firefox's RAM use.
The program couldn't be any simpler to use. Download it, unzip the file, run Firemin.exe - and you could stop there. No installation required, no settings to configure, Firemin simply runs in the background, looking for the Firefox.exe process and, if it's there, uses Windows API calls to reduce the browser's memory consumption.
It worked for us, too, drastically reducing RAM consumption.
But there is a catch. Firemin must run all the time (while you're using Firefox) to do its work, and Windows also has to spend extra time swapping RAM out, and bringing it back when necessary.
Fortunately this probably won't make much difference. And if there's an issue you can address it a little in the program's options, reducing Firemin's CPU usage at the expense of accepting extra Firefox RAM consumption.
Firemin will instantly save you more than 100MB of RAM, and maybe much more, but also increase your hard drive activity and CPU usage. It may still be useful if you're short on memory, though - try it and see