Hello Paulaexum, DON'T do that for a while, keep it safe somewhere. The "Old Firefox Data" it is something like "back up" for your old profile.

Note: Your old Firefox profile will be placed on your desktop in a folder named "Old Firefox Data." If the reset didn't fix your problem you can restore some of the information not saved by copying files to the new profile that was created. If you don't need this folder any longer, you should delete it as it contains sensitive information.

copied from the next link, please it is useful to see the article : https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

thank you

See also:

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile

You can use this button to go to the currently used Firefox profile folder:

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