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.