The "Old Firefox Data" folder is created when the browser is refreshed. It contains the original profile you were using before the refresh.

If anything seems wrong or missing, you might be able to recover what you want from it. Once you are sure you no longer have need of the old profile, you can remove it if you want.

Thanks, Fred.

Glad to help. Safe Surfing.

What initiates a 'refresh' Fred. My old Firefox file seems to be following what I think is the real on. The old currently has 118MB and is dated 25 July 2019. I have had a lot of bother with MS updates recently. I do like to keep all files if poss because of dementia probs.

See this https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings for info about what the Refresh feature is about.

User initiated from a dialog box / window were Firefox recommends that a Refresh be done.

As far as what would trigger to that recommendation of a Refresh be done goes, typically when the Firefox program "thinks" that a "fresh start" with a "new Profile" is due, due to 'age' or other parameters. That can happen when an older installation of Firefox is used after being dormant / unused for quite awhile; and probably when the latest version of Firefox is installed with an update.

And recently with Fx 67 / 68 with the enhancement of a new feature that allows for an automatic creation of a new Profile when a 2nd, different version of Firefox is installed; Firefox can now use the version specific Profile for each version that is installed for that Logon User for that computer.

Thanks for the very helpful reply. As I only use my PC for genealogy & e-mails and all appears OK I will proceed to delete the 'old' one. PC does appear to be runnings faster with the updates, I only hope that in a few months time one of the obscure history sites I occasionaly use is still with me! Thanks again for your prompt help.