When you reinstalled Windows, did you choose to save your data? Open your user profile. Look for a folder called Windows.old

I've forgotten my firefox password, so I had to change it.

If you are talking about sync;

Resetting the sync password, or changing its e-mail will remove all data from the servers. The username and password are used to create the encryption.

This is a security feature. https://support.mozilla.org/en-US/kb/ive-lost-my-firefox-sync-account-information

• I've lost my Firefox Sync account information - What to do
• I’ve forgotten, or don't have access to, my Firefox Account email address

Note: Only the data on the servers is cleared. The profile data is not changed or removed.

I don't have Windows.old folder.

Ye, I was talking about bookmarks from sync. I remember that after every reinstall I just had to sync Firefox and I had my bookmarks back.

But as I posted, changing the password will remove all data from the servers.