Do you happen to have a folder on your desktop called Old Firefox Data or something similar? If you do, see the Recover important data from an old profile documentation for more information on recovering your profile data.

If you don't have that folder, it's possible that Firefox created a new profile for you during the update and disabled the old Firefox profile. If that's the case, your old Firefox profile may still be on your computer.

Find your Firefox profile folder (see Profiles - Where Firefox stores your bookmarks, passwords and other user data for more information) and see if there is more than one profile folder listed. If there is, let us know.

NOTE: You are running Firefox Beta. Although Firefox Beta is the most stable development build, it's still intended for testing purposes only since there may be minor bugs that still need to be worked out.

Thank you so much for responding Wesley. I now have the problem fixed and have backed up all my bookmarks onto a thumb drive. Not going to risk losing them again :)

I know others have had the same problem so your answer will no doubt be helpful to them too. Thank you again.

No problem. Glad that I was able to help.

Did you end up having an "Old Firefox Data" folder on your desktop or did you have a secondary profile that you recovered your data from?

I actually did find the Old Firefox Data folder. I wasn't able to find an earlier secondary profile.

Your help was much appreciated  :)