If you set that to True in Firefox 1.5 (it may be in earlier builds, I know it is in 1.5.0.1 for sure), it will tag memory it uses as 'flushable' on minimize for Windows. It can/will make it a little more laggy when you unminimize the process but it is great for having a lot of browser windows open and then launching a memory hungry app like Quake 4.
For instance, unminimized, FF was using 50MB as an example, and minimized it was using about 2MB of RAM. I don't know how well that 'scales' yet but it is a nice feature if you tend to leave a bunch of browsers open in the background.
Note: It isn't there by default as a toggle option. Easy way to put it there is to open up a "about:config" window, right click within the window, and select "New Boolean" and then enter in "config.trim_on_minimize" and set it to 'True'.
You could also stick it in your profile preferences, but this way is the easiest to describe.
Edit: It seems to make a big difference. One Firefox session had around 600MB of memory use drop to 20MB when minimized. It also seems to work for Thunderbird, which is nice because my RSS threads have been bloating the memory footprint and I typically leave that minimized anyway.