HI Rogersaurus, There is a small hack you can try. In a new profile, or the default one, open about:support.

1. From this page there will be a column "Profile Folder", click on "Open Folder"
2. Now that you have the location of the folder, you can make a copy of the profile folder that you backed up from the previous computer. However make sure that you copied this folder path from the previous computer:

Profile Manager - Create, remove or switch Firefox profiles

Also see: Back up and restore information in Firefox profiles

It is usually best not to copy the full profile folder, but only restore files with personal data that you really need like the

• bookmarks and history: places.sqlite and the backups in the bookmarkbackups folder
• other SQLite files like cookies.sqlite (Cookies) and formhistory.sqlite (saved form data)
• logins.json and key3.db for the passwords.
• permissions.sqlite and possibly content-prefs.sqlite for the permissions and Site Preferences
• cert8.db for stored intermediate certificates
• sessionstore.js for (pinned) tabs

Extensions are best be installed anew and well as toolbar customizations and Options/Preferences settings.

Rogersaurus said

I have just obtained a new pc which runs Windows 8. I had saved my old Firefox data so I could restore my bookmarks and add ons. However, once I had installed firefox on the new pc, it wouldn't show a new folder in which I could copy my old data. Refreshing didn't help as, when I re-open Firefox, it does not give me the option of using old data. Please help!

I've actually managed to fix the problem myself by going into Profile manager and creating a profile. If anyone else has the same problem, happy to help.