There is no definititive answer, even the devs admit it... some people experience it, some dont... most of the time tho, its because youre using something that firefox just doesnt like, which includes flash, java, etc.
check around www.neowin.net , theres an official thread regarding memory leaks... a big reason for memory "leaks" so far is because firefox keeps a cache of your recently opened tabs/ sites so your ram is filled with pics, sites, etc.
I don't know for sure but I've always heard that the leak occurs mainly when you open multiple instances of the browser. If you keep one window (with or without multiple tabs) it's supposed to not leak as badly. As always, YMMV.
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