an item - https://hacker-news.firebaseio.com/v0/item/8863.json
a page (new,best,ask,show,jobs) - https://hacker-news.firebaseio.com/v0/topstories.json ..(this returns an array of ids, you'll have to iterate that and make separate requests for each entry.) Also these are limited to 500 items.
a user - https://hacker-news.firebaseio.com/v0/user/jl.json
max id - https://hacker-news.firebaseio.com/v0/maxitem.json (you can use this to iterate all items or update all items since a last cache)
updates - https://hacker-news.firebaseio.com/v0/updates.json