Firefox can export/backup bookmarks in 2 different formats. What is the file name of the backup file for your bookmarks. The file name will normally have a .html or .json suffix.

1. JSON format is normally used to backup the bookmarks. When restoring from a JSON file it will replace the bookmarks with those in the backup.
2. HTML format is normally used to export the bookmarks, and this format can be read by most browsers. If you import an HTML file, the bookmarks are added to the existing bookmarks.

For details of how to restore from a JSON file or import an HTML file see Backing up and restoring bookmarks - MozillaZine Knowledge Base

Firefox stores bookmarks in a file called places.sqlite, was it this file that you backed up rather than using one of the export/backup options of Firefox?

Make sure that the file with the bookmarks has the correct file extension: .html for a HTML backup and .json for a JSON backup.

If you are not sure about the file type then you can open the file in Firefox via "File > Open File"

A JSON backup will show as one long text line without line breaks and a HTML backup as a web page with clickable links.

@ Quarantine Its neither JSON nor HTML file.Its of unknown format. I backedup using backup options of firefox. The problem is that that file doesnt have any extension.

@ Cor-el I opened up and it shows exactly a JSON backup file shows but then its not a JSON file. Im too confused... Should i post the whole long text line..???

You can only post the first par, 100 or 200 characters is enough to identify the file.

A JSON backup starts with ({"windows":[{"tabs":[{"entries":[{"url":
If your backup starts with that sequence then try to add .json file extension
To make sure that windows doesn't preserve the original file extension you need to add quotes around the name: "bookmarks.json"

Here's the text

{"title":"","id":1,"dateAdded":1278523297695000,"lastModified":1278706191140000,"type":"text/x-moz-place-container","root":"placesRoot","children":[{"title":"Bookmarks Menu","id":2,"parent":1,"dateAdded":1278523297695000,"lastModified":1281337284484000,"type":"text/x-moz-place-container","root":"bookmarksMenuFolder","children":[{"title":"Recent Tags","id":2006,"parent":2,"annos":[{"name":"Places/SmartBookmark","flags":0,"expires":4,"mimeType":null,"type":3,"value":"RecentTags"}],"type":"text/x-moz-place","uri":"place:sort=14&type=6&maxResults=10&queryType=1"},{"index":1,"title":"Recently Bookmarked","id":2005,"parent":2,"annos":[{"name":"Places/SmartBookmark","flags":0,"expires":4,"mimeType":null,"type":3,"value

ab90,

That's a .json file. Here are the first few characters of one of my bookmarks-YYYY-MM-DD.json files from {profile folder}/bookmarksbackups:

{"title":"","id":1,"dateAdded":1271681951296000,"lastModified":1271682730031000,"type":"text/x-moz-place-container","root":"placesRoot","children":[{"title":"Bookmarks Menu","id":2,"parent":1,"dateAdded":1271681951312000,"lastModified":1281106340000000,"type":"text/x-moz-place-container","root":"bookmarksMenuFolder","children":[{"title":"

A JSON file needs to have a .json file extension, otherwise Firefox won't recognize it as such.
You may need to add quotes around the name ("bookmarks.json") to force a new file extension on Windows.

@ Helper Its not the Json file as cor-el said u need to have the Json extension and it doesnt have that extension.

@ Cor-el How to get that Json extension attached to this file.

Try to add quotes around the full name including the .json file extension ("bookmarks.json") as I wrote above to force that new name.

If that doesn't work the try to rename in a cmd.exe window (Start > Run) with the rename command
Make sure that you are in the correct folder, if necessary then use cd "path"

rename "old_name" "new_name.json"

ab90,

My post about the format of the file being a .json file was building on cor-el's post in this thread Posted August 17, 2010 5:11:05 PM PDT

"...then try to add .json file extension. To make sure that windows doesn't preserve the original file extension you need to add quotes around the name: "bookmarks.json"..."

Thank you for posting this answer. I had the same problem with my backup being an unknown format. It wouldn't allow me to add quotations around it, but I was able to look at the properties (Windows XP) of the bookmark file and type in a new name with the .json extension and that worked like a charm! Thanks!

Thnx cor-el and helper it worked.. Now i have a new problem :D ive traced up my previous bookmarks and many have been restored.. Now i need to merge both of these lists... Any solution??

Restoring a JSON backup will replace all your current bookmarks (you get a warning about that), so if you already have bookmarks that you wish to keep then export them to an HTML file before restoring the JSON backup.

A way to merge bookmarks is to use two profiles and restore a JSON backup in each of them. Use Copy in the Library (Bookmarks Manager) of the second Firefox instance to Copy selected bookmarks to the clipboard and Paste them into the Library of the Firefox instance opened with the default profile. You need to start the second Firefox instance (profile) with the -no-remote command line switch.

Another way is to restore the first JSON backup and copy all the restored bookmarks to the clipboard.
Then you can restore the second JSON backup and paste the bookmarks that were saved on the clipboard to merge the previous set.

Cor-el ur a genius man... Thnx ..!! =)