i'd rather have my memory working than not. as long as you do not have any problems, out of memory errors, freezing, bsod's etc., don't worry about memory useage. cpu useage is another story. what version of FF may be significant if it's one of the early ones.
the old memory bugaboos from win95 and 98/me are no more. winxp on up handle memory differently and more usefully. as memory is needed a program is assigned it. when it's not needed anymore, it is not released in case it's needed later, as it's faster that way. if another program needs memory, it will however give it up.
free memory is memory that is unused and not working for you. why buy gigabytes of memory and then try your best not to use it? if you really must, go into the config (type 'about:config' without the quotes into the address bar) and change config.trim_on_minimize to true. when you minimize FF it will release any unneeded memory. it may then need to reapply for it when you restore it, which will take a bit more time.
do not get taken in by the hype from some software that 'frees up memory'. those programs and their overhead are not needed and are in fact counter-productive. they usually work by forcing the program to page the extra memory to the page file, where it then takes much longer to read back into the program when it's needed again. you are better off by letting windows do it's job. you are better off buying more actual memory. ( i note your specs do not say how much memory you have, as a percentage of the physical memory i suspect from the other spec of your system that 200mb is not a very large %)
task manager does not give a complete picture of memory usage anyhow. try procexp.exe (process explorer) from sys internals (now part of microsoft), it's x86/x64 and gives a better breakdown, here's my firefox details: i'm running the 3.6 beta 6 version.