Please ... any help is much appreciated !

I know pretty much nothing about Sync, but if something is gone, it's gone.

Did you not sync both phones before removing Firefox from the old one?

Do you have bookmarks synced? Would those not contain most sites you had open in tabs?

If you are missing a lot of stuff, it cannot hurt to reinstall FF on the old device, provided you did not delete the user data, or you have a backup you can restore (e.g., from Google Backups, or if your phone vendor did backups, or you have manual backups, etc.). Fire it up, connect to Wifi, and let it sync again. [I would suggest turning off sync on the old device until you are finished and sure it has the data you want to sync.] I have no idea where tabs are kept, i guess in session data.

Best wishes with all that. Sorry a Sync expert isn't answering.

Synced tabs expire after a week, so if this is from longer ago then your tabs are lost.

Thank you VERY MUCH for both of you cor-el AND for the advises Unfortunately crankygoat, I tried your tips and I couldn't find my tabs back ...

cor-el is right about the 7 Days sync expire

Thank you !