You can't just replace the Profile, the name of the old Profile won't be recognized in the new Firefox installation. You need to copy the contents of the old Profile into the new Profile folder replacing the files in there. or rename to old Profile folder to that of the new Profile folder, or edit the profiles.ini file to change the new Profile name to that of the name of the old Profile.

I just wanted to thank you for posting an answer so quickly. I was so excited to get my new pc, and then when I thought I had lost all my bookmarks (which I hadn't even bothered to write down), I was so disappointed. I had to try a few different times / ways, but I did finally get Firefox to recognize my profile files. You really made my day! Thank you!