8.4.2.7 is the latest FTOS available for the S25. I am not aware of any set date for new firmware release date. Is there something specific you are trying to accomplish that 8.4.2.7 does not support? Or just trying to stay current? 
According to the Force10 support site it is not supported as of June 2015.  Does this mean there is currently no supported FTOS for my switches?  The next scheduled release is "To be announced."  When can I expect to see that announcement? Does anybody know anything about this?
Force10 S25n - Is 8.4.2.7 truly the latest version of FTOS available?