Tools/Account Settings/Synchronisation and Storage
Checkbox 'Keep messages etc". Look in Advanced and check the folders you want.
Repeat for each account you have.
I also use Mozbackup to easily make a backup file.
OnePlus 5T review: Hands-on
Illustrator Andrés Lozano on his improv line work, brazen use of colours & hand sketching
iPhone X review
Comment envoyer gratuitement des gros fichiers ?