More questions...
Which version of FF are you using?
Have you recently installed a newer version of FF over the top of an older version? If so, try uninstalling and reinstall latest version. You won't lose your settings and Bookmarks by doing so.
I think there was an issue with FF using a lot of memory when minimised, but I thought later versions had rectified that. Nevertheless, it could be 'resting' in the memory, to enable a quick restart.