Your gmail and imap.gmail.com are a bit odd. I've had those too and I don't know where they come from.

You might try visiting the gmail website, log into your email account there, and remove the labels. And then delete them in Thunderbird.

As for [Gmail]…

I have several gmail accounts and they all have this sub-structure. When I meet something like this, I ask myself:

• who put it there? (it wasn't me)
• what's it for?

If I don't know the answers to these, my inclination is to leave well alone. The subfolders beneath [Gmail] include Spam and Bin. Now, where would spam and deleted messages go if these labels/folders weren't available? What if messages are incorrectly categorized as Spam, or deleted by mistake? How would you recover them?

You can flatten the file system here by setting [Gmail] as the top-level folder in this account. Go to Tools|Account Settings|{select account}|Server Settings→Advanced|IMAP Server Directory and set it to [Gmail].

I haven't tried this in anger myself. I tested it once and it appeared to work, but I really couldn't find a justification for such a change and possible ramifications.

I use Saved Search folders to aggregate multiple similar folders (e.g. Junk/Spam/Bulk Mail, or Trash/Bin/Deleted) across multiple accounts, so I don't tend to care where folders are actually located.

These problems were noticed around the time Windows 10 was installed, but that comment doesn't help. I find the Gmail options over-complicated especially as I only want the basic heading for the Thunderbird bookmarks. I have 4 Gmail accounts and they all seem to have a list of bookmarks of their own.

They are folders, or labels in Gmail terminology. I find it odd that you consider them to be "bookmarks". "Bookmarks" usually means placeholders set by the user for his own convenience and these things in Thunderbird appear to be somewhat more permanent and persistent. If Google choose to put things into a specific folder then that's what we have to work with.

This is how Google choose to implement IMAP-based email. I've had this argument before that the user should have the ultimate choice, and the answer then is to go re-set the base folder, and grow your own folders effectively within the [Gmail] folder thereby rendering it invisible.

You have two choices; change the account settings, or work with it as it is.