FF saves the pages that are in a tabs history in RAM. (for example, when you press back, it loads the page from memory)
so the more browsing yo do /tabs you have open, the more ram it consumes.
For example, I have 22 tabs open, of which about 10 have fairly deep history queues- FF is consuming 596,116 KB.
However- even closing the tabs doesn't help- because it saves the tabs you've closed, too. (probably just hides the buttons). This is why it constantly rises. there is a "browser.sessionstore.max_tabs_undo" that can be changed with about:config that might help to reduce RAM usage from getting too high from a large number of tabs.