You need to export the font, not save it.
Here's an explanation, quoted below for posterity:
To save files, simply use File->Save as . This saves the font being edited as a *.sfd file.
However, to get an actual usable font, you must use File->Export . In our situation we will be mostly exporting Truetype fonts (which have the GSUB and GPOS tables). However, Fontforge can export in a variety of other formats - take a look at the official documentation for a description of all the supported formats.