Do you want to restore it to Firefox, or just look at the contents of that file?

The only use of a JSON backup is to restore the file via the Bookmarks Manager in a Firefox profile, that will replace all current bookmarks with the ones in that JSON backup.
A JSON backup is a text file in object format and is not very readable.
If you want a readable backup then export the bookmarks to an HTML file.

Why do you want to open that file?

Just wanted to look at the contents to make sure that the file is complete before I copy it to another place. Thanks, cp

Thanks, html solved my problem. cp

I want to extract the tag information along with the bookmarks. I can't do that from the HTML version. Can I do it with json? Is there any documentation of the bookmark file json format?

HTML bookmarks files don't contain the Tag data. The Tag data is in the JSON bookmarks backup file.

See this Google search of the Mozilla Developer Center for JSON documentation.

I can parse JSON files in general, but the one with my bookmarks is 745K. I could poke around in it for a week before finding what I want. Is there some place where the structure of this particular file is documented? How do I associate URLs with tags, given my JSON file?

Never mind, I figured it out.

@anewc2 Could you please give me a hint on how you figured it out. I have the same problem and I can't make heads or tails of my JSON file without several days of cutting and pasting... Thank you.