FWIW, I saw exactly the same memory creep a month ago.
I saw it in the daily report, and after scratching my head realized that it correlated with an open session, after I closed the browser the memory usage dropped to normal levels quite quickly. I left it logged in again for a few hours and it started creeping again.
I believe it was on a WinXP sp2 unit with the latest FF but my recollection is fuzzy.