I have several computers, each with its own iteration of Thunderbird. Just like Chrome and Firefox do for settings and favorites, why can't the address book in Thunderbird be synced so no matter which copy of Thunderbird I open and use, I will have access to all address book entries I've made across all iterations? This is the worst part of using Thunderbird, the frustration of not seeing addresses from another computer's Thunderbird. I really don't understand why this flaw is allowed to exist.