open Thunderbird, from the troubleshooting information on the help menu, select the show profile button. Close Thunderbird. Windows file manager will have open showing you the current profile folder Thunderbird is using. It should contain a prefs.js file. Open your profile backup folder and locate the prefs.js file in the backup. Once this is showing in the file manger press Ctrl+A to select everything in the folder, all files and folders. Then press CTRL+C to copy everything that is selected. open the profile folder where you first saw the prefs.js file. Make sure to click on the folder name so it is selected. Press Ctrl+V, that will initiate a paste of your old profile. This should start raising prompts about replacing files, lots of them. Select the do not prompt option when our sick of being asked to replace a file. If you are not prompted to replace files, the paste is going to the wrong place. once the paste is complete, start Thunderbird. It should "just work"