Thanks for the tutorial. There are a few things that may be worth pointing out about using this setting.
First. if you don't wish to edit about.config you can create a file called user.js and place it in your firefox profiles directory. edit the file an enter the following line:
Save the file and restart firefox. Essentially any configuration changes you want to make to aboout.config can be made by entering the appropriate setting in user.js.
Second. making this change does not actually reduce the amount of memory fx uses, it just swaps memory from RAM to disk. From the Mozillazine KB:
setting (Windows only) does not reduce the amount of memory used, it just swaps memory from RAM to disk. This may make Firefox slow to "wake up". If config.trim_on_minimize is left set at the default value (false), the memory will be swapped to disk only if it is needed by another application. Therefore, for most purposes the default value is recommended.
There are also other settings which also play an important part in controlling fx memory, please look at:
Finally you may also want to look at the Ramback utility: https://addons.mozilla.org/en-US/firefox/addon/5972
Hope this helps.