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I managed to resolve this issue, although I don't use sub-folders so I'm not sure if this procedure will work for that, although if you add a top-level folder as per the instructions, the ones under it should follow by default with no extra configuration needed.
Go into your Gmail account through a web-browser and sign in to set up the IMAP folders you want visible.
Go to "Settings" (top-right corner of Main Gmail Pane), then go to the "Labels" tab. Under "Labels" (halfway down the Labels Pane and on the left) type something like (for example): [IMAP]/All Mail in the box that says: "Create a new Label" and click the "Create" button.
Use whatever category (ies) names from the top part of the "Labels" pane you want to see in Thunderbird. Make sure to put the square brackets [ ] with 'IMAP' between them in all capitals, then the forward slash / ,then the foldername wanted (spaces are ok, ie 'All Mail' works). Watch the caps and small letters - they have to match identically to the names in the top-pane to work, also there are no spaces in the [IMAP]/... part, run them all together.
Then, log out of gmail (logout is important), (close Thunderbird if it is open, also important) open Thunderbird, and the folders should show up under your gmail account name in the accounts pane on the left (Unified or All Folders view). One more quick hint: in Thunderbird, if you right-click on any folder under your account-name (in the accounts-pane on the left) you should see "Subscribe"; left-clicking on that will show you all the folders Thunderbird can see based on it's most recent conversation with the IMAP server, so make sure the folder you want is there and check-marked to be active (They usually are by default when they are added), if your folder is not there, try closing and reopening Thunderbird, maybe wait (should be no more than 30 minutes) and retry closing and opening it. Some of mine took a while to show up the first time. If it still isn't there, sign back into Gmail from a browser, because there is probably some error there: spelling, spaces, caps or small letters, etc....
I hope that helps.