The folder is created by running the Refresh feature, sometimes called a Tune-up or Reset. Your entire old profile folder was moved into Old Firefox Data for reference in case you need to restore anything. For more information on this feature, see: Refresh Firefox - reset add-ons and settings.

Do you recall running a Refresh manually, or allowing a Tune-up when Firefox suggested it? Are you missing any critical data, settings, or add-ons?

Think of it as a backup of your data. I've only seen that after doing a refresh.