The more tabs you have open in firefox, the more the web shield will be scanning. This really is normal behaviour, you brows, web shield scans and windows is basically incharge of memory control, e.g handing back memory if needed by other applications.
How much RAM do you have left (as 65mb from 1GB is a drop in the ocean), because it is there to be used or you end up slowing your system down as what is in memory has to be swapped out to the pagefile.sys. If it is subsequently required it has to be swapped back into memory, this is much slower than if it remained in RAM (and why windows doesn't hand it back quickly) and it also thrashes your hard disk.
So I would suggest that you audit what is running in the background, and only have essential applications running and if that leaves little in the way of RAM left then you need to get more RAM, it is real cheap now and another 1GB would be great in XP Pro. It is what I use and have 2GB and I never have to worry about RAM.