It is a well known fact that Firefox can consume quite a bit of memory during normal usage. If you are so inclined to see where/how this memory is being used, simply visit the following URL:
about:memory
This works in the newer versions of Firefox and shows, in great detail, the breakdown behind the usage. If you are curious about what a certain column means, you can Google it for a quick answer.
The main number to look at is the top value "Memory mapped" which is the amount of memory (physical + virtual) which Firefox is using. This value will match the "Commit Size" in Windows Task Manager.

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I have some dirty memory! How does Firefox know what websites are ‘dirty’ and what are websites are not!
Thanks for the link. It is quite useful.