Although I had gotten a report via a web site comment that Mozbackup didn’t work with Windows 8.1, that is not my experience. Mozbackup is free software that backs up Firefox and Thunberbird profiles.
I just finished using Mozbackup (version 1.5.1) to install my Firefox bookmarks from my desktop computer to my new notebook. It was quite easy.
All I had to do was use Mozbackup to back up my Firefox profile on my Desktop computer, which runs Windows 7. Then, I could copy the backup file to my notebook using my network or even a flash drive. That’s "could."
Actually, I shared the backup directory on my Desktop, then ran Mozbackup on my Win8.1 notebook, and told it to restore from that file.
Mozbackup gave me the options to restore General settings, Bookmarks, History, Extensions, Saved Passwords, Cookies, Saved form details, and Certificates. I unchecked all except Bookmarks.
Some of the others might have been ok, depending on my intent, but the most significant thing to remember was that I was taking a Windows 7 backup and installing from it onto a Windows 8.1 computer. I didn’t want to cause any issues of possible incompatibility of Extensions(Add-ons) or settings within Firefox.
A quick Google search for Mozbackup will provide links to the author’s site to download it. Mozbackup is free.