You can use this button to go to the current Firefox profile folder to see what location is used.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Firefox uses two locations for the Firefox profile folder. Location used for the main profile in "AppData\Roaming".

• C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>\

Location used for the disk cache and other temporary files in "AppData\Local".

• C:\Users\<user>\AppData\Local\Mozilla\Firefox\Profiles\<profile>\

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

See also:

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

My old Firefox bookmarks are in this folder and it plays the error sound :

C:\Windows.old\Users\Alexey\AppData\Roaming\Mozilla\Firefox\Profiles

Did you check the bookmarkbackups folder in that location?

What do you mean with playing the error sound?

You need to copy the places.sqlite file to the current profile folder or use the most recent compressed .jsonlz4 backup to restore the bookmarks in the Bookmarks Manager (Library).

• Bookmarks -> Show All Bookmarks -> Import & Backup > Restore
• http://kb.mozillazine.org/Import_bookmarks