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After upgrading it became apparent that the "Send" but does nothing when replying to a mail. All settings in Gmail Conversation was adjusted with no success.
Using Xubuntu 14.04
for this. Also the "to:" field is showing "please wait" forever.
An update, I could find no workaround, so only solution was to disable gmail conversation at this point.
+1, same symptoms
The newer version is pending approval from addons.mozilla.org ; you can download it from https://addons.mozilla.org/en-US/thunderbird/addon/54035/versions/ in the meanwhile.
Sorry for the inconvenience,