Lots of folks are complaining about the so-called "Firefox memory leak", but it turns out there's actually a decent explanation for what's going on. Still, if you want to quickly reduce the amount of memory that Firefox is using, you could try this technique, first posted by Ryan at This May Help Your Firefox Memory Leak. Note this only works on Windows, and that if something goes screwy it's not my problem. Other than that, it appears to really work.

1. Open Firefox and go to the Address Bar. Type in about:config and then press Enter.
2. Right Click in the page and select New -> Boolean.
3. In the box that pops up enter config.trim_on_minimize. Press Enter.
4. Now select True and then press Enter.
5. Restart Firefox.

If you want to be careful, go to Ryan's page above and read the comments. Try it out - if it doesn't help, just reverse the process.

(And yes, I wrote a book about Firefox - Don't Click on the Blue E!: Switching to Firefox - that you might find interesting.)

(Check out all of our posts on Firefox.)