How long since you used Sync on the old computer? Sync isn't intended for log term storage of Firefox data, but rather for synchronizing Firefox data across multiple devices as Firefox is used with day to day use of Firefox.

Did you happen to change the Firefox Account Password? If so, your data is gone. Data is encrypted thru dual passwords that are generated from the userset password. Different passwords can't decrypt data that was encrypted via different passwords; thus that useless data is deleted.

As far as recovering data from the old computer goes, pull the hard drive and you can use something like this - https://www.amazon.com/Dotop-Adapter-Converter-Optical-External/dp/B002OV1VJW - to connect it as an external drive; and then recover your data from the old hard drive.

Thank you for your answer Indeed I change my password !! :-( (it will be great if for once they put a popin with a little message to say that you have to loose your preferences ahahah)

I've got time machine history from my old computer but I open the Firefox package but I didn't know how to restore ?

Thank you very much