The custom firmware is an addition to the standard firmware, and provides additional facilíties. These facilities are (in the main) confined to providing remote management of the Foxsat via a web browser across your home network, and access to the recordings on the Foxsat via the home network (including streaming recordings to DLNA-capable clients). Recordings can be copied to (for example) a PC by FTP or network file access (as if the Foxsat were a NAS).
StDef recordings are not encrypted. HiDef recordings are encrypted (and cannot be decrypted), but can be recorded without encryption by a hack called "Nowster's Patch". Without Nowster's Patch, HiDef recordings are of no use except playback on the original recording device.
The Foxsat custom firmware has a long history, starting as simply a means to access the file system but then incorporating the web browser interface developments from the HDR-FOX custom firmware. As such, the user interface and capabilities are similar (but for technical reasons not the same) as the web interface for HDR-FOX.
The relevant information about the Foxsat CF is summarised in this topic (see particularly post 2):
http://hummy.tv/forum/threads/networking-and-modified-software-guide.896/. Note that the main support forum for the Foxsat CF is on AVForums here:
http://www.avforums.com/forums/free...e-server-bundle-foxsat-hdr-release-4-0-a.html.
The HDR-FOX CF (on which the Foxsat web interface is based) is thoroughly documented on this forum, to get a taste of it see
http://wiki.hummy.tv/wiki/Custom_Firmware_Overview.