If the old system was Windows you can copy your old profile to your new profile after wiping out the content within you new profile. You can restore all of your application files, not just Firefox, and make a mistake, you can start over or whatever it takes, nothing to lose right now that you haven't already lost or misplaced everything so have everything to gain from what you recover. I recovered all of my data from my old laptop it took awhile, it's a lot easier to work with the old stuff if you can still access the data. I certainly left some things off of my backups. If you've been backing your files, you also have the opportunity of seeing whether you backed up everything you needed -- kind of sounds like you are relying on the old drive, at least keep track of what you restore. Kind of the opposite of what you are doing now, see :How do I save my bookmarks to a flash drive? | Firefox Support Forum | Firefox Help :https://support.mozilla.com/en-US/questions/856495