We see this issue with almost every release of Firefox, but only with select users. There are millions of users who get Firefox updates without any issues, but there are still a handful that have issues. It's not the intended behavior of Firefox, since that data should be saved. If you see a folder on your desktop called "Old Firefox Data" or something similar, you can recover your information from that. See the [[Recovering important data from an old profile]] documentation for more information.