Open a new tab and enter about:config in the location bar, this should bring up a massive list of preferences, but be careful with it. Anyway, enter browser.cache.memory in the filter, and check that browser.cache.memory.enable is set to true, and check if you have browser.cache.memory.capacity, if not right click and choose new > integer, and name it browser.cache.memory.capacity
I have my capacity set to 16000, which stops firefox getting out of hand.
You might also want to check the values of browser.cache.disk.enable and browser.cache.disk.capacity too, and if you are still having troubles browser.sessionhistory.max_total_viewers tells firefox how many pages to keep in memory, try setting it to a number between 1 & 8, or 0 for 0 pages cached. -1 lets firefox choose itself.
Hope that can help you.