Did you export the bookmarks as .html from the older version before trying to import them? From which version did you update? Was it from a version that saves bookmark backups as .json files, or as .html? Did you wipe out your old profile, or did you allow the new version to use it? Have you checked the bookmark backups? If you did not wipe those out, you may be able to restore bookmarks from there.

Can you open the HTML file in Firefox for inspection via File > Open File ?

See also:

• http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox
• http://kb.mozillazine.org/Import_bookmarks

I exported the html file first, then opened it in the new browser. It worked.