I've created my own JSON bookmark backup according to this page : http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox#Creating_bookmark_backups
I won't post my JSON bookmark backup file here (it's too big), you can create your own file and have a look at the entire file.
Then for testing I just tried to get the uri of all bookmarks (later I will extract other datas too) but this didn't work
jq -r '.[] | .uri' bookmarks-2017-09-13.json
jq: error (at bookmarks-2017-09-13.json:1): Cannot index string with string "uri"
jq -r '.uri' bookmarks-2017-09-13.json
null
Version of Firefox : Firefox 55.0.2 (64 bits) with Ubuntu 16.04 LTS
Version of jq : jq-1.5-1-a5b5cbe
Regards