...doesn't have a file extension...
Have you unticked "hide extensions for known file types" in folder options?
But we are not quite right here if its FF 3.6 in question.
.json files are backup files used with the restore function.
.html are the bookmark files used with the import function.
Select the restore function for your .json file instead of import.
Or maybe you can you just export your bookmarks to a .html file which is the easiest way to save a copy.
If this does not work for you, drag and drop your file into the Firefox browser. This should display your bookmarks as a page of links. If it does not, the file is either broken or something different so then we are stuffed.
If the file is ok, untick hide extensions for known file types and rename your file giving it a .html extension.
Then use the import function.