Firefox is a Memory Hog (150+mb)

Windows 7 had a complete re-engineer of its memory management. And more tweaks were done with Windows 8. Inactive programs not being used get pushed to the background and while they are allocated the memory shown in Task Manager, they aren't actually using it until they are brought back to the front.

How is your PC running? Is it sluggish? Are mouse movements jerky? If its running fine, I wouldn't "task watch". Just use it and enjoy it.