First, set Windows to show hidden files and folders and file extensions: https://support.microsoft.com/en-us/windows/file-explorer-in-windows-ef370130-1cca-9dc5-e0df-2f7416fe1cb1 - See these sections at the bottom: * "Show file name extensions" * "Show hidden files" Next, explore up this path on your old drive, which I'll call "E" for explanatory purposes: E:\Users\<username>\AppData\'''Roaming'''\Mozilla\Firefox\ Here you should find at least 2 files and 1 folder: * installs.ini file * profiles.ini file * Profiles folder The two .ini files have information about which profile (in the Profiles folder) was your default. However, you probably can tell from just from looking at the "Last Modified" dates for files in the individual profile folders which has the latest data. Once you identify the one you want to reinstate, please see the steps in the following reply: https://support.mozilla.org/en-US/questions/1474421#answer-1687884 - your source folder will be the Profiles folder on the old disk instead of the Old Firefox Data folder mentioned in that reply