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When you open multiple main messenger windows in Thunderbird, Google Contacts attempts to synchronize from each of them.
This causes all sorts of issues, such as multiple synchronizations happening at the same time, errors in one window when you close another window in the middle of syncing, Thunderbird hanging or getting very slow because of the overlapping synchronizations, etc.
You need some sort of semaphore to ensure that synchronization only happens in one window at a time. This is complicated by the fact that the user could start with window A which is the "active" window for synchronization, then open window B, then close window A, at which point (but not before) window B needs to know to take over and start doing synchronization.
You may have seen bug reports from users about Google Contacts creating duplicate contacts. I believe that the scenario described above, i.e., when multiple windows are doing synchronization at the same time, is what causes this. I recently had 500 duplicates of one contact and 472 of another in my address book and gmail account! I'm not certain about this; it also could have been due to the fact that I was synchronizing multiple TB instances against Google rather than just one, or due to the fact that I was also synchronizing Outlook against Google with gSyncIt.