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#11
Pop & imap
Thunderbird will work better if you use IMAP. As far as having more than one version it's best to uninstall Thunderbird completely and start afresh with the latest version. Sometimes Thunderbird can screw up and a clean install is the best approach.
You can leave POP enabled in Gmail if you want to but for Thunderbird it's better to use IMAP so enable it in your gmail account settings.
Once you've removed all trace of Thunderbird - rebooted and then reinstalled it there are quite a few settings that will help it to work better than the default settings.