Gzipped JSON is now the standard format for transferring memory report data. But diff-memory-reports.json can't handle the "gzipped" part, so you have to manually gunzip the files first. This is pretty lame. I also wonder if I should rewrite the diff tool in Python. I wrote it in JS because I figured that would allow it to be incorporated into Firefox easily in the future. But maybe that'll never happen, and until then you need a JS shell build to use it, which is something of a pain.
I'm going to integrate the standalone diff tool into about:memory in bug 857382.