I would vote against this feature. If you have a lot of changes to be synced and are on slow internet connection, syncing can take minutes.
Consider that you need to reboot your computer: In this case waiting minutes for firefox to quite would not be considered a feature, but a bug by most users.
If you have chosen "sync automatically", Zotero synchronized very frequently. (If it does not, then there is a problem wit automatic sync that needs to be addressed)
Define "frequently" :D Last time I had almost 20 references and two notes not synced (half an hour?), they're not lost but it's annoying if you work on several computers.
Consider that you need to reboot your computer: In this case waiting minutes for firefox to quite would not be considered a feature, but a bug by most users.
If you have chosen "sync automatically", Zotero synchronized very frequently. (If it does not, then there is a problem wit automatic sync that needs to be addressed)