Thats it Simon,
every time you do a new install the "profiles" folder has a different file name such as "9eht674h.default" or "zk946crb.default"
So when trying to do a restore with that program, it cannot find the correct "profile" to restore to. But once you have transplanted you existing "profile" into the newly installed Firefox version, your sorted, as it will then be able to restore correctly. Of course doing up grades to Firefox or Thunderbird does not affect it as your "profile" folder file path does not alter.
So, though your right to back up your "profile" folder as a "fail safe" measure. Lets say, for those who wish to semi auto back up their email folder say on a weekly/daily basis to another hard drive or partition. And have a "restore" facility as well this program is handy.