If those bookmarks are in HTML format, you can import them into Firefox. Import appends existing bookmarks.

If you have an old places.sqlite file or a json backup file, you should Export HTML... your current bookmarks before proceeding.
Bookmarks > Manage Bookmarks (labeled Library) > Import & Backup
Then restore a json backup file through the Library, which will replace your current bookmarks with the old ones.
Or, replace your current places.sqlite file with the old file - located in your Profile folder.
http://support.mozilla.com/en-us/kb/Profiles#How_to_find_your_profile
Then your need to import your current bookmarks, which will append them to the the old bookmarks that you restored.

Thank you. I'm a bit confused as to why importing something I've saved to a JSON file would append the imports while leaving the existing ones alone, while importing what I have on the disk would wipe out my existing bookmarks. What I have on the disk is also a JSON file; when I click on it I get a long string of characters.

I found my profile, and it appears I have several JSON files there (six, each with a date this month) in a folder labeled bookmark backups. Opening these, they look identical in format to the old one on the disk. So wouldn't I be faced with the same problem (replacing one with the other) if I replace the new with the old? (I.e. when I then try to add back the newer ones I'll be told it'll wipe out what's currently on active bookmarks?