Do you still have access to the old device or have a backup? *https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile Firefox uses two locations in the hidden "AppData" location for the Firefox profile folder. Primary location used for the main profile that keeps your personal data (Root Directory on about:profiles). *C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>