Note that Sync isn't designed to be used as a backup, but is merely meant to sync data between connected devices. You can copy certain files with Firefox closed from the profile on the old computer to the profile folder on the new to transfer personal data. Note that best is to avoid copying a full profile folder. For the bookmarks you can copy places.sqlite or use a recent compressed .jsonlz4 backup in the bookmarkbackups folder. *https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:profiles</b> page (Root directory). *Help -> More Troubleshooting Information -> Profile Folder/Directory:<br>Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data <hr> *bookmarks and history: places.sqlite *favicons: favicons.sqlite *bookmark backups: compressed .jsonlz4 JSON backups in the bookmarkbackups folder *cookies.sqlite for the Cookies *formhistory.sqlite for saved autocomplete Form Data *logins.json (encrypted logins;32+) and key4.db (decryption key;58+) for Passwords saved in the Password Manager<br>key3.db support ended in 73+; to use key3.db in 58-72, make sure to remove key4.db *cert9.db (58+) for (intermediate) certificates stored in the Certificate Manager *persdict.dat for words added to the spell checker dictionary *permissions.sqlite for Permissions and possibly content-prefs.sqlite for other website specific data (Site Preferences) *sessionstore.jsonlz4 for open tabs and pinned tabs (see also the sessionstore-backups folder)