The config.trim_on_minimize 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.
The RAMBack extension, available for Firefox 3, allows the user to manually free memory, such as caches, that is usually used to increase performance. This gives the advantage of small cache sizes without having to always suffer the performance hit.