There might be files which you deleted but but are still on disk.
This happens if a process still has the file open.
You can check for open files by doing:
sudo lsof +L1 firefox 15940 klu txt REG 0,39 130896 0 4064267 /usr/lib64/firefox/firefox (deleted) firefox 15940 klu 12r REG 0,39 9698845 0 4064383 /usr/lib64/firefox/omni.ja (deleted) firefox 15940 klu 13r REG 0,39 12591502 0 4064263 /usr/lib64/firefox/browser/omni.ja (deleted) firefox 15940 klu 37r REG 0,39 3172 0 4064259 /usr/lib64/firefox/browser/features/e10srollout@mozilla.org.xpi (deleted) firefox 15940 klu 38r REG 0,39 678258 0 4064260 /usr/lib64/firefox/browser/features/firefox@getpocket.com.xpi (deleted) firefox 15940 klu 39r REG 0,39 1072 0 4064261 /usr/lib64/firefox/browser/features/webcompat@mozilla.org.xpi (deleted) firefox 15940 klu 71r REG 0,46 577860 0 8468126 /home/klu/.local/share/gvfs-metadata/home (deleted) firefox 15940 klu 75r REG 0,46 32768 0 8468127 /home/klu/.local/share/gvfs-metadata/home-e0bec4b0.log (deleted) firefox 15940 klu 76r REG 0,46 514204 0 8444625 /home/klu/.local/share/gvfs-metadata/root (deleted) firefox 15940 klu 84r REG 0,46 32768 0 8444626 /home/klu/.local/share/gvfs-metadata/root-45cc9117.log (deleted) firefox 15940 klu 90u REG 0,41 655360 0 432729576 /tmp/mozilla-temp-1470880785 (deleted
Now, all these files are still on disk, because firefox has them open. Close/restart firefox, and the space will be released.