Lost saved files when switching to Firefly 3.

@HRMNY. I will start by asking, what were you using before, and is the previous executable still on your computer?  If so, you might be able to still run that version and recover the data.

If not, generally, this type of data recovery is not simple and might be impossible.  The browsers store this data in application support directories. On Mac, that is in the hidden directory "~/Library"; Windows has a similar structure. Digging in here is not recommended for people unfamiliar with this.  Your browser has a specific directory in this structure that contains the cache and configuration files. It is possible that it generated a new profile when you upgraded but the old one is still sitting there.

The other option is, if you have backups of that data, you could restore from backups.  But most people do not have backups. I have configured my Mac to back up that area with Time Machine, so I can restore my browser profile with the loss of about an hour.  Several times I have been saved from a disaster by being able to roll back my configuration.

If you have someone in your friend group or in your area that is an expert in your operating system, that might be you best path forward.