I have started using gmail thru Thunderbird and every incoming email is duplicated 3 times (it will appear once in the 'Inbox' folder, once in the 'Gmail/All Mail' folder and once in the 'Gmail/Important' folder).
This is annoying because I have to open the same email 3 (once every copy of it) times to ensure that the notification icon doesn't signal three unread mails. So I have to always go to the 'Gmail/All Mail' and 'Gmail/Important' folders to mark folder as read every-time I get a new mail.
How can I remove the 'Gmail/All Mail' and 'Gmail/Important' folders in Thunderbird (obviously without deleting the content of 'Inbox')? Is this something that can be fixed from within Thunderbird, or do I have to change something on the webmail side?
You can also tell gmail itself to not present the relevant folders to the IMAP user (thunderbird) at all. To do this, go to Gmail > Settings > Labels, find the relevant labels, and untick "Show in IMAP".
Unsubscribing the "important" folder can resolve the issue.
This can be done by right-clicking any folder in the inboxes panel and clicking "Subscribe...". That will load all folders Thunderbird is currently watching, and you can untick the box to unsubscribe from "Important".
You can find further information on imap folders and subscription in this superuser quesion: What does it mean to “subscribe” to an IMAP folder?
The answers above no longer work for T'bird 68.10.0. The trick is to click on the account name -> Manage folder subscriptions -> select the folder you don't want and uncheck its box. The Important and All Mail folders are under [Gmail].
Even easier is to right click on the account name -> click on Subscribe..., then as above.