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.
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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)