1. Open the Library window (Bookmarks->Organize Bookmarks)
2. Press Import and backup
3. Choose Restore
4. Click Choose file...
5. In the dialog box that pops up, select the JSON file on your system and open it.
6. A dialog box will ask if you want to replace your current bookmarks. Choose Yes.

If it doesn't work after following these steps, try the solutions at Can't add, change or save bookmarks - How to fix.

I got my bookmarks back. Thanks heaps zzxc!

zzxc's technique works. A few notes:

• Firefox regularly creates a .json backup of your bookmarks in the "bookmarkbackups" folder in your profile.
• In Firefox 5 (at least on Linux), the menu item that opens the Library window is Bookmarks > Show All Bookmarks.
• This 'technique replaces all your bookmarks.

If you want to add bookmarks to your existing bookmarks (for example, you were using a different copy of Firefox and want to add the bookmarks you made in it), you need to import an HTML file of bookmarks, not restore this JSON file. Here's one way to do it:

1. Create a new Firefox profile (see steps elsewhere) and use it. (It should have no bookmarks except for the default Firefox set.)
2. In that new profile, import the JSON file as zzxc explained. (It should now have the bookmarks.)
3. Then in Library, choose Import and Backup > Export HTML... and save to a file. Give it a descriptive name like 'temp_setof_bookmarks_to_import.html so you don't get confused
4. Exit that Firefox profile, and restart Firefox using your normal profile.
5. In Library, choose Import and Backup > Import HTML... and import the HTML file you created.
6. You'll now have both sets of bookmarks. You'll probably have duplicates of the default Firefox set.