this is what FF sync is intended to do. however, i have no confidence in FF on the Mac's. so if possible, i would not do any fancy things with your ff.

if you do want to sync with other devices, be sure to execute a sync and monitor the little green spinning wheel to ensure syncing it is not active a the time you log out. then log out. then on the other device, log in and monitor the syncing wheel. after it stops, then log out.