Something is wrong with my system setup on SSD. During the installation of Ubuntu 14.04 I selected "use entire disc" for my 256 GB SSD.
Now after reload fstrim
shows 200 GB trimmed and just after opening and closing Firefox with several tabs fstrim
clears another 600 MB:
(I have 200 GB of free space and haven't deleted anything. Partition table is set to MSDOS (default). On the previous system installation (same version and same hardware, but manual partitioning) everything was fine.