You cannot restore your history from the bookmark backups, just the bookmarks. If you mean the bookmarks will not restore, perhaps the places.sqlite file in the profile is damaged.

• http://kb.mozillazine.org/Locked_or_damaged_places.sqlite

I miss spoke / typed. There is no import, restore or backup option in the library window. The following does not work / I don't have these options when I do what it says.

(Quoted for here) "Restoring from backups Caution: Restoring bookmarks from a backup will overwrite your current set of bookmarks with the ones in the backup file.

   At the top of the Firefox window, click the Bookmarks menu and select Show All Bookmarks to open the Library window.
   In the Library window, click the Import and Backup button and then select Restore.
   Select the backup from which you want to restore:
       The dated entries are automatic backups.
       Choose File... lets you restore from a manual backup (see above). 
   After choosing a backup, your bookmarks from that file will be restored. Close the Library window. "

I FOUND IT!!! Apparently in Ubuntu the options in the top tool bar only appear when your cursor is on it. It's all there & I got my bookmarks restored. Thank you & sorry.

At what stage do you run in to a problem

1. open bookmarks library (Ctrl+Shift+O)
2. click
   1. import and Restore
   2. Restore
   3. Choose File
3. navigate to your .JSON backup files
4. stop - have you made a backup of any existing bookmarks that are about to be overwriten - do so now, or continue
5. click on to highlight and select the file, then click on the [open] button.

The posts crossed, glad you sorted it out.

Note with Ubuntu you still may use the Firefox Button, if you choose not to use the file menu toolbar.

The option to use the top toolbar that appears on the edge of the screen occurs when you use the default add global menu bar integration