If you're not a developer or don't know what a memory leak really is, then stop using that phrase when you're describing Firefox's memory usage/management. Stop it. Unless you are a developer or advanced technician and have actually identified a real memory leak, stop using that term.
First, you need to become familiar with the fact that the numbers reported by Task Manager ("Mem Usage" and "VM Size" to name a few) are not a realistic measurement of the memory being used by a process.
It is important to know that to improve disk performance and to lower disk thrashing and paging, Firefox no longer frees the memory used by previous site renders and opened tabs. This way, if you visit a site again or use a resource that was recently used, it is immediately ready. Also, by hanging on to a huge window of memory, it is available to Firefox on demand without Windows having to page out some other program to disk to make room for a newly opened tab. The result is visible performance. Tabs and windows open/close faster, etc, and disk thrashing is very minimal.
They fixed one "problem" of disk thrashing and paging to curb some users, and others see it as a huge drain on system resources (which it's not).
The memory that Firefox is holding onto is not going to be freed unless it is necessary. If you open Photoshop (or something similarly huge) and start doing filters on a 300 DPI 3000×3000 pixel image, you'll see Firefox get paged out immediately, and the memory it's holding onto will drain away. If you are not doing anything other than using Firefox, why /should/ Firefox free the memory? Your system doesn't have xy MB of RAM for not using, it's for using. The more memory you have, the more memory Firefox will allocate. To claim that the fact Firefox is using 200 to 500 MB of your system RAM when you're not doing anything else and have 1GB+ RAM is a memory leak is just out of control.
Why does it seem like Internet Explorer uses far less memory? IE uses Windows' default memory management method, which I'll call the "low performance/disk thrashing" model. It's also important to note that half of Internet Explorer's dependencies are already in memory due to the fact that they are also used by Explorer, and other parts of the shell.
You should also take a hard look at the extensions you have loaded in Firefox, as some of them can be downright buggy. Tabbrowser Extensions (not to be confused with Tabbrowser Preferences), for instance, is a pretty unstable memory hog, known to cause actual memory leaks. Replace an extension like this with a far superior one, such as Tab Mix Plus.