Did you have any problems when trying to log into Sync in the new operating system?

Like maybe you needed to reset your Sync password?

No i did not any problem in login but in problem in to lost data

Did you remove the Firefox Firefox profile folder when you uninstalled Firefox?

Do you have an "C:\Windows.old" folder?

If you have a "C:\Windows.old" folder with personal data from a previously installed Windows version then you can recover personal data from a previously used profile in this backup.

• C:\Windows.old\Users\<user name>\AppData\Roaming\Mozilla\Firefox\Profiles

You can copy files like these to the current Firefox profile folder to recover specific data.

• bookmarks and history: places.sqlite
• bookmark backups: JSON backups in the bookmarkbackups folder
• SQLite files like cookies.sqlite (cookies) and formhistory.sqlite (saved form data)
• logins.json and key3.db (decryption key) for Passwords saved in the Password Manager
• cert8.db and possibly cert_override.txt for (intermediate) certificates stored in the Certificate Manager
• persdict.dat for words you added to the spelling checker dictionary
• permissions.sqlite for Permissions and possibly content-prefs.sqlite for other Site Preferences
• sessionstore.js for open tabs and pinned tabs and tab groups

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

New thread here - https://support.mozilla.org/en-US/questions/1128571 - about changing the "mozilla" password in each of two separate devices and then losing the data on each.

i dont have mozilla old file.