Specifications of firefox bookmarks json file?

[bobmct]

I'm trying to create a bookmark maintenance utility that uses the above reference file as its input. While I have no problems opening and separating the fields I've been searching for, unsuccessfully, to find a detail description of what each element represents. Of course I can figure out some of them but there are a couple that elude me (i.e.: the 'index' field).
Can anyone please point me to the specifications/DTD for this file?

Much appreciated, thanks.

[Malric]

I guess that's more of a question for Firefox devs. Sorry.