What steps are you taking when you try to restore that file?

Make sure that the file has a .json file extension.

The only solution I have ever found for moving all the info from my existing Profile is to copy the existing default file, as is, somewhere, then once the install or upgrade has been made, delete the 'content' of the new default file and paste and copy the entire 'content' of the old default file to the new one. It is the only solution I have found for Firefox and Thunderbird to accept old my files.

I must confess that I simply don't understand why this can't made easier for Mozilla Users - I have to do the same with Thunderbird ... drives me crazy each time. Why can't we have Import last default info and or bookmarks, etc.

See also http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox

With my solution I get my Firefox up and running with everything that was saved before the re-install, i.e. Bookmarks, History, Scrapbook, etc.

indebele,

Yes, your solution does work, but there is an easier way to restore a json bookmark backup file, which is what this thread is talking about.

Bookmarks > Organize Bookmarks -> Import & Backup > Restore = Choose file...

"Import" is for bookmarks.html files, the "Restore" is for json backups.

The Mozbackup program can be used for backing-up and restoring Profile data for both Firefox and Thunderbird. http://mozbackup.jasnapaka.com/