Tried downgrading Firefox to the latest version in the official update repo - no joy, same memory leak. Tried downgrading flash player to the previous version - memory leak is gone. So, it seems this is a memory leak in the latest flash player update. Downgrading the latest available flash-player-11.2.202.466-2.58.1.x86_64 to flash-player-11.2.202.466-2.55.1.x86_64 resolves the issue.